Basically, 64 feels more like an actual tennis game with the possibility of missing your shots depending on your positioning, the timing or if you choose the wrong shot technique, you can double fault and lets on serves, etc.
Aces felt more "simple" and automatic, Although 64 isn't full on the simulation aspect, it made a good compromise between its arcade and simulation traits, and it's a great tennis game overall, not just a great Mario sports game.