Just to add to the discussion...
I tried Mari and it choked on a 3 million poly mesh....so its a no go for me as even if Im doing gameart I like to paint the hipoly and then bake both color and normal map simultaneously. What Mari seems to be great for is vast amount of textures and very big sized textures.....which I dont need.
I like using Mudbox and then use 3DCoat clone brush on a slighly lower subdlevel,I use 3DCoat clone brush because its the only clone brush in all painting apps that does not sample the screen but the Uv sheet and I think its a pretty extraordinary V4 feature.I also do all my shader baking in 3DCoat as its much more straightforward and powerful than 3DS max "render to texture". Also 3DCoat got tons of little texturing tools that both Mudbox and Mari dont have...so most of the times I send a lower subd from MB to access those great tools....In a sense I work in Mudbox but I use 3DC as if it was Photoshop.
I think 3DCoat painting is really optimized for gameart (layered 4k/2k maps) and it gives access to interaction with normal map that is still unbeaten.
And vertex painting is fantastic for 3DConcept art which a lot of studios are doing more and more nowadays in Zbrush...I think if those people would be aware of how powerful 3DC vertex painting is they would do it in 3DC instead of Zbrush.A lot of people dismiss vertex paint as being useless I think they are just unaware of of much it is currently used in production at the design/concept stage because of its great adaptability to art direction changes.
I dont think 3DC will ever be able to compete with Mari regarding film industry...for ex:adding support for painting animated meshes would be pretty absurd to implement in 3DC IMO...I mean those kind of features are probably developed by entire teams of programmers and Mari is entirely dedicated to this kind of stuff anyway....it would be like asking Mari developers to add voxel sculpting or retopo tools...just my opinion here .