Zach and I had a conversation about this, as I am trying to plan my long term poker future online. It boils down to basically the "rakeback" in the form of VPP bonuses is much MUCH easier to attain on Pokerstars
than it is on Fulltilt. The Iron Man promotion is a real joke.
For us cash game players, it's also about table selection and availability. There are more players on Stars, thus at the limits above 50nl, there are MUCH more games to choose from.
Also between both sites, I have found at 25nl the games are MUCH looser at PokerStars. That's contrary to public opinion but I am just looking at my database numbers ... although my fulltilt hands outweigh my pokerstars hands 20 to 1.