Aren't you about a month early for making such decisions?
Anyway, the way I'd go about it is to do it in two steps. First, turn operator inputs into a conceptual trio of values: forward/backward velocity, left/right velocity, and rotation rate. The way you do that depends on what OI controls you use. Then, compute the individual motor speeds from the intermediate values. That conversion depends on what kind of wheels you use and how they're mounted.