There's not any true difference between the paint in the Model and Game ranges, it's in the names on color formulae. The Model colors have been around forever in 250 or so colors, I believe, designed for historical wargaming with colors to match the historical ones. They became aware that many mini painters had found their paint and felt it to be superior to that of GW, but many of those painters still wanted GW
colors, if not the
paint itself, since those colors were still the gaming industry standard -- ie, they wanted to paint their Dark Angels and Blood Angels in Dark Angels Green and Blood Angels Red, with guessing which Vallejos they should be trying to use instead. So Vallejo released the Game Color line which was the same paint as the Model, only in colors that specifically matched GW's colors exactly.
This site, by the way, shows a table to match both Vallejo lines to GW. The Game colors will be exact matches, while the Model colors will be the closest equivalent as gathered from a consensus of mini painters around the net (which, in most cases and to many eyes, will still be pretty damn close). Bottom line -- doesn't really matter which Vallejo line you use.
Sorry for the length, but I hope this helps.