Stunningly beautiful Phi Phi is still as popular as ever. There are no cars on the island so it's an altogether different place.
The main decision is weather you want to be around other people; on a stunning beach with a resort and not much else or in a busy village that's sandwiched between two beaches. Ton Sai doesn't have the same seclusion but offers many shops, restaurants and bars and a little nightlife if that's what you want.
Why not split your time between the two, either way Phi Phi is great place to stay all year round.