It depends, what games you are playing. For tournaments I think,
888 Poker is way better than Stars. Lower rake, softer games and smaller fields makes it much easier to win money on 888 Poker, if you play tournaments. For SnGs Stars is way better though, because there is just much more game selection and activity. For cash games Stars is better for 2NL and 5NL due to lower rake, whereas from 10NL and up it does not really matter, which site you play. Also, as someone said, rather than thinking about it as "moving", why not think about it as having an account on more than one site?