My modeling tool box has 5 brands of paint in it. There is Games Workshop as well as GW Foundation line, Vallejo Game Color, Vallejo Model Color, and Privateer Press P3 paints. Since both GW paints and both Vallejo paints have some significant differences I count them as separate brands. I will also be talking about what I consider to be the 3 different types of paint in each line. That is Basic, Metallic, and Inks/Washes. None of the lines are equal in value across those types so I can address them separate.
Vallejo Model Color
I will start off by talking about Vallejo Model Color or VMC. This is the paint that I use the most by far. It started off as simple emulating of talented painters that led me to VMC. They used them so they had to be good, right? Well they are. The pigment is some of the finest you will fine. That means that as you thin the paint it holds color to a thinner consistency. Since thin coats make for a smoother finish this is incredibly important. Then there is the bottle. I have a love hate relationship with my Vallejo dropper bottles. They are handy when dispensing paint because you can drop out one or two drops and then close the bottle back up. You are not leaving the pot open to the air to dry out as you use it. They also make it really easy to measure mixes because you can handle all the ratios in drops. So a 1/1 or 50/50 mix would be one drop of each color. The also dry flat with little or no sheen but are not chalky like some of the cheaper craft paints. In addition the colors in this line tend to lack any pop! They all lean towards the conservative side, and are not that vibrant. This is also one of the largest lines out there, but there are many colors that are extremely close to others.
As for the cons, for me there are only a few. The first is really a convenience issue. VMC paints are rather quick to separate in the bottle. That means that shaking them is a must pretty much each time you use them. You can add an agitator such as a BB (can rust) a air soft pellet (some are too light) or even ceramic beads to help facilitate mixing, but they can prove a challenge because they have a tendency to fall to the tip when you turn a bottle upside down to dispense paint. That leads to my least favorite thing about the bottles (remember earlier I said love/hate). The bottle tips have a bad habit of clogging. This is easily remedied by keeping a stick pin nearby, but that can be a hassle when painting to stop and unclog the tip. Finally a finishing coat is almost a necessity for VMC as it does not have a hardener added in the same manner as the other four do.
The strength of this line is in the basic paint. The metallic paint does not cover as well as other brands which can be a blessing or a curse. If you are really working a model it is good as it makes for a smooth coat if laid down in thin coats. If you are just looking for table top quality, there are other brands that are better. As for the inks, they really only have two. They don't work in the same fashion as others they are more glazes than washes or inks. They don't have strong enough pigment content to settle and shade as well as others washes do.
Vallejo Game Color
The next one I will talk about is Vallejo Game Color or VGC. This paint shares a lot of the same good traits as the VMC has. It also has the benefit of an added hardener. This gives the paint a slightly more durable finish but that comes with a bit of a satin finish. To me that is not a problem because I painted with GW paints for over 10 years and it was never a problem. Plus I normally spray my minis with some sort of dull coat and that all but kill any shine it may have.
I have some of the same issues with this paint as the VMC such as the dropper and the separation. Added into that is the fact that the hardener causes the paint to dry a bit faster, but that can also be a good thing in some applications. The vibrant colors that are missing from the VMC line are hiding in this line. They have named many of their colors to match GWs colors.
The basic paint in this line is strong just like the VMC paints. The inks leave a lot to be desired as they just don't have a strong enough pigment content to settle properly for washes/inks. As glazes they are great but that can easily be accomplished with thin paint. The metallic paint is pretty much on par with other brands. They cover well and thin about the same as others. One bright spot in my opinion is the "Polished Gold". This is one of the better covering gold paints out there.
Games Workshop Foundation
Games Workshop has been doing some good things with their paint line the last couple years and Foundation paint is one of them. This is some of the best covering paint I have used. Even when thinned it covers fairly well. The colors are formulated to be a base for other work so they tend to be a bit.....bland. That is not a bad thing as a coat of a foundation can actually save you 2 or 3 coats of other colors. This paint has a hardener just like the VGC so it is durable. The one main downside that I experience with this paint is that it seems to have a threshold where at one point it covers fine, but any thinner and it takes forever.
I almost never use this unless I am working on large projects. Vallejo makes colors to match and I prefer them, so the main benefit with foundation is coverage and that equals speed. That is what you gain.
Since this line only has basic colors there really is nothing else to talk about. Rumors were flying when they came out that GW intended to make metallic paint in foundation that has yet to materialize.
Games Workshop is the paint that most gamers know. They have a decent size line with some great colors. They cover almost as well as the foundation paints, but are not as drab. They have a fine pigment and thin well. They also hold together well and rarely separate. This paint is a great basic paint and most painters will never find any real benefit from upgrading to another brand. They have a similar hardener as other brands but I find them to be the most durable of the lot. This really is only a problem for other brands until you finish coat your models as that should hold your paint in.
There are many cons for GW paints to me, but none of them are game breakers. They recently upgraded their pots to the same type as the foundation paint, but if you have the older pots like the one in the picture they are extremely prone to drying out. I have seen many painters go through extravagant measures such as store them upside down (my approach) to storing them in zipper bags to keep them wet. Its was trivial but a problem across the line. It also is a bit faster drying than other brands but this is not a huge issue. The flip top pots make transferring paint to a pallet to work a little more difficult, but this could just be my prejudice for the dropper bottles.
The basic line for GW paints are as solid as any. The metallic paint on the other hand are by far the best as a whole. They have a fairly fine metal flake size, and for the most part they cover well. The washes are another star for this line. Although they behave in a completely different fashion to their washes in the past in all reality it is an upgrade. They can be a one coat solution that works similarly to "dip" methods that others use with out the heavy coverage those polyurethanes leave. The one downside to the washes is they don't settle as dark in the depths like others and they cause a complete change of tone to the original color when used.
Privateer Press P3The Privateer Press P3 paints are probably the paint I am the least familiar with. I bought a Menoth faction paint set to see how they work, and before I got a chance to really put them through their paces my A.D.D kicked in and my attention went elsewhere. What I do know from what little I use them is they are a fairly solid paint. They share the stability that GW has in that they rarely separate. They also share the similar consistency and finish as GW paint does. The colors in the line tend to be a little less vibrant than GW or VGC paints are, but they are pretty much suited to match the factions as PP portrays them.
In sharing some of the benefits of GW paint they also share a few of the problems. They do dry a bit faster than the Vallejo colors do. Once again this is not always a problem, but something of note. The pot design is similar to pots that GW used to use. Unfortunately they also share the same flaw. The tab on the front of the pot has the tendency of breaking off and that can make them difficult to open. The hinge on the back also tends to break and that leaves the lid free. At the time I bought my paint it was only available in sets and that was a turn off for me, and I am unsure if that has changed as of writing this.
The basic paint in this line is solid. Good coverage, durable. It is not something I would switch to GW for becuase it is not all that different. The washes I have tried share a lot of the same traits as the VGC inks and are and area where they deviate from GW. The pigment saturation is thin so they don't show well in the depths of the model. The metallic paint is also a point of departure for their paint. Granted I have only used a few colors (two golds and brass balls) and I was fairly disappointed in their coverage.
Let me restate my notice from the beginning. I am in no way any type of expert when it comes to paint and brands. I am simply responding to his question and hopefully you can take something useful away from it.
Having said that, GW and P3 paints are extremely good brands of paint. They handle everything you really could ever want to do with a mini. I just have a personal preference to the Vallejo lines for most things. Take note though, my assortment of paint has at least a few of each of the above brands and they all see some use. So if someone were asking me for advice I would suggest go with what you like. If you are curious about a different brand, buy a few pots and see how they fare.