on phuket side at that time of the year, you may catch the tail end of the rainy season, i was out there last november/december and then in january (my wife is thai and lived in phuket).
the vis around phi phi is not going to be any where near as good as the similans, check my web site for some pictures of what to expect around phuket/phi phi.
marina divers do 3/4/5/6 and day trips to the similans there is a link on my site.
most of the other dive places use marina as they have the boats, so it is best to go straight to them.
one warning though, do not rent a bike, as you are responsible for ANY damage!
i have told the story before:
my brother hit a swedish woman who walked out in front of him, he fractured her skull (needed several drain holes drilled!), 5 ribs (one of which punctured her lung), pelvis, wrist and she stopped breathing at one point and needed zapping.
this cost him in excess of £1500 to buy his freedom back from the police (x1.5 for $).