I don't know how you plan to drive your car so it is hard to answer this question, but aluminum being lighter will let you wind the motor up faster. On the other hand you will have less quick slowdown on compression when you lift off the throtle. It is really a matter of what you want and how you plan to drive the car. I don't think you gain much for normal street driving or crusing with the lighter rotating mass but for racing you will gain some.
Ron
