Both are great guns and spare parts are readily available. Both have their little quirks, but overall neither of them is vastly superior to the other. Go for the one you find the best looking or most comfortable. If you plan to use the iron sights, the MP7 is WAAAAAY better. If you plan to use an optic, then it doesn't matter at all. Mags are the same price for both, and both could use an upgraded hop-up system (Maple Leaf EVO thingy, see this thread for a very succint usage report:
Vector stuff - Airsoft Canada) .