I've used both the Game and Model colour in the past. Basically the model colour has a much, much bigger range and the concept behind it was for people painting historical miniatures (largely without mixing paints) - although don't let that put you off using them as some of the colours are fantastic! The Game colour (in my opinion) is Vallejo's attempt at mimicking the Citadel Colour range of paints. Personally I prefer Citadel as they now have a much more consistant finish to them, which for miniature painting is really what you're after. I tend to use a handful of VMC colours for specific things - that said I can pick up a pot of Citadel paint much more easily than I can a pot of VMC so for me that's also a consideration.
pete