If you want to get lots of info on this question do a search; there's probably 10 or more threads about this debate.
I started with Entourage, and it's fancy, cool, etc., but also really buggy. Got to where it would beachball quite a bit, then finally it wouldn't download my hotmail, which it had been doing just fine.
Switched over to gmail, and to the Mail app, and no more problems. I'm probably not using it to its fullest potential, but soon will start doing more in the way of scheduling, etc., with Mail, iCal, and Address Book. The three work fairly seamlessly.
Also has something to do with what kind of mail you have, and will the program support it. Good luck!