4* as I'm enjoying that much more then 5* in WE7. I'm going to try 5* on PES3 first, however, to see if there have been any changes since WE7.
I think for people who have not previously experienced WE7 or the PES3 BETA versions, having some exhibition games in 3* will help a lot. I remember starting a game in 5* when I first got WE7 against AC Milan. I got thoroughly shafted 4-1. However, now, with lots and lots more games I can play on 5* well but I find the game more satisfying on 4* and just as much of a challenge.
There's been some talk that 3* star is more defensive then other modes and it's harder to score then in 5* or 4* but I'm not too sure on that. It isn't the case with me, anyway.