IMO until you reach stakes where you rake a decent amount, you should stick to sites with RB, like iPoker. Reason: check how much RB you can get on Stars now (I would be surprised if this would be 25%). Then compare it to 55-60%. Still plenty of fish there, providing you are familiar with table selection (I hope you are).
Rakeback should be considered as extra bb/100 to a winrate, so if you're being raked 8bb/100 then 60%rb vs 25% rb is like an extra 2.5bb/100.
Player pool softness/volume and software etc should be taken into consideration. Stars almost always has good games so I'd think most people would get in a decent amount more volume (you're not going to log on and see 8 tables full of regs) + have a more enjoyable experience.