Pokerstars #1, no doubt...
888 have a lot of bonuses, "free" chances etc
Right now I play on both, but I prefer and will always prefer PS because I feel so relaxed there, people are ok, I feel somehow good at PS.
888 will give you ton of chances to win something, but in most cases, if you dont deposit any money, you can almost always win few cents up to few dollars every single day (depends).
They have ton $0,01 cent tours with $0,10 tickets/$1 tickets and so on, ton of chances for beginners to build their bankroll step by step.
Anyway, PS is, was and will be #1 option for a long time.