The correct grip size is important for control and comfort. The best grip size is the one that matches your hand size. See here
for an explanation on how to find the right size. I have extremely big hands and need to order the 4 5/8 (the largest size) and put three overgrips on!
Sorry I can't give you any advice about the racquet. I use the ProKennex Ionic Ki5 and haven't tried a prestige. You probably already know most of this, but here
is a page about selecting an appropriate racquet given your game and requirements.