One financial advantage is that deposting on a new site will earn you some sort of deposit bonus.
Of the sites I have tried:
Betfair - reasonable opposition but rakes seemed, at lower stakes ,a little high c.f. other sites so don't play there any more.
Party Poker - relatively easy site to win money at both STT and ring games - good promotions and monthly deposit bonus.
Full tilt - don't really like the interface which always seems slow to me - multi-tabling is tricky as well. Good variety of games such as Horse and Razz which do not appear on a lot of sites.
Bodog - good freerolls
and the best deposit bonus. In terms of players, there seem to be more chasers or people who think middle pair is a great hand at this site. I think this is due to the sportsbook connection this may or may not suit you. In STTs & MTTs, if you play tight, this could potentially work against you but in ring games the easy money that can be won with a good hand is a definite advantage.