Vallejo Games line is specifically designed to mimic GW paints - you can find charts online that tell you what VG color equates to which GW color, but you can probably figure it out on your own. Its also nice for old guys like me because you can get some nice colors in VG that GW has discontinued. On a side note though, a lot of painters don't like the VG metallics - the mix and look is largely agreed to be inferior to GW metallics. So if you can afford it, get the metals direct from GW.
Vallejo Model Color is a more muted, subdued line designed for more traditional model builders. The paint is the same good quality, but it is indeed less "flashy" than the game color. GW paints are designed to really "pop" on the tabletop - it goes along with the exaggerated features on their miniatures and the extreme highlighting painting techniques used by the 'Eavy Metal team. Model Color is more for the guys painting WW2 tanks in true historical colors. That being said, you are correct that the Model line is much more extensive than the Game line, so if you want the extra versatility then mixing the two lines is a good idea. I've seen it done on 40k minis very successfully.
So my advice is: get the Game Color box, supplement with Model Color as desired, and get your metallics from GW.