I think both are so good (in realism) that is difficult to choose, the differences you feel might come more from being simulations of different cars than from the simulation being better or worse itself. And of course the base game matters, the MMG don't have (at least out of the box) the force feedback of the reiza, but that is not the fault of mak corp.
As for which one is more fun to drive.. TBH, if you want fun in race07 you take the f3000, and in gsc the f3. They are nowhere near as realistic (maybe the f3 is), but I enjoy sliding them around heaps more.