Hi Michael,
It really sounds like you have been unlucky in Phuket and lucky on Samui. Normally the visibility is quite good, both during the low season and the high season, and quite often better than in the Gulf of Thailand.
Starting February the vis can get less at the Similans (normally a bit later in the year though), due to the growing amount of plankton in the water. But than with the plankton the manta rays come, last year already half December and in great abundance. Again later in the year, normally April, there are whalesharks, but you have to be really lucky to spot those.
The daytrips to the Similans both from Khao Lak and Phuket are nice. The Similans are quite far though, so that either means lots of travel time for 2 dives or traveling by speedboat, what is not my personal preference. Also the boats operate on a fixed schedule, so it can happen that when you arrive at a divesite, there are already divers from other boats in the water. On a liveaboard normally the boat can switch sites, on the daytrips they don't.
Plenty of operators in both Khao Lak and Phuket, so simply make your pick.