Both are good options for different reasons. The big sites, pokerstars, full tilt, ulitmate bet, are good because you get game and table selection as well as huge toureny fields with gigantic prizes. If you are primarily playing cash games I woudl suggest maybe signing up on some smaller sites. If you are not playing any draw poker I would suggest signing up with full tilt poker as they have some decent deals out there. If you play tourneys than I would suggest pokerstars or full tilt.

Honestly I have made money on quite a few sites, big and small, you just have to understand the draws of a large site vs a small site. While you might get a great bonus on "pokerworld.com" good luck finding a good game to play in while at the same time you can grind out 1000's of hands at 20 different tables on pokerstars without getting a bonus and still do realy well.