I generally agree with Jamie, but I also warn that the monsoon can come a bit earlier than is usual in the Gulf, and it'll ruin your late October plans if it does. Last year, the last week and a half to 2 weeks of October were just awful on Koh Tao/Koh Samui/Koh Phangan. There were many unhappy vacationers who spent their long-awaited holidays watching the rain pour down and splashing around through the flooded streets, with 2-3 meter waves often blowing out the better dive sites and 2-3 meter viz underwater on those days when the boats could go out.
If the weather's OK in the Gulf, then absolutely do not miss Sail Rock and Chumphon Pinnacle, because both can be truly first-class sites on an average-to-good day; a handful of other dive local sites (White Rock, etc.) are worth visiting too. If you're into diving deep wrecks, then contact Jamie and he'll set you up with whatever tech diving needs you might require (a couple of the wrecks are at recreational depths if you're willing to do very short no-deco dives).
In general, I'd say that the macro life is more diverse on the west coast; I'd also say that you've got a better chance of seeing lots of pelagics (big jacks, tunas, mackerels, barracudas, etc.) at those afore-mentioned sites in the Gulf than at most sites on the west coast, although some west coast sites offer frequent sightings of pelagics both large and small. E.g., you won't likely see mantas any time in the Gulf (whale sharks are always possible at a number of sites, though, and there are usually grey reef and bull sharks at Chumphon Pinnacle), but in Feb. the mantas are literally guaranteed on a Koh Bon/Similans liveaboard trip, and there are often some amazing schools of rainbow runners and various other big trevallies and related predators feeding on huge baitballs off Koh Haa and some Similans sites, with big trumpetfish waiting discreetly in the wings for their own chances to dart in and gobble up a meal.
On the west coast the Similans and Koh Bon and Richelieu Rock are the most famous -- and mustn't be missed -- but I'm also quite enamored of some of the more southerly sites such as Hin Daeng, Hin Mouang, some of the Koh Haa sites, etc. (Koh Lanta is a great place to stay and a good launching-off place to dive these sites, which as of a couple years ago, when I was there, were every bit as good as the Similans). At these sites you can see mantas, whale sharks, leopard sharks, mantis shrimps, etc., and if you look carefully there are loads of macro critters like ghost pipefish, Durban dancers, various nudibranchs, etc. ... I haven't seen the southern west coast sites since the soft coral disease hit (an effect of that red tide?), though, so I can't speak to their present condition. West coast sites should be diveable by early - mid-November, depending upon the timing of the monsoon.
Note that the west coast lacks a lot of deep wrecks like there are in the Gulf: there's a nice recreational-depth ferry wreck between Phuket and Koh Phi Phi, the King Cruiser, with loads of marine life on it (and loads of divers, too), and a quite shallow and beat-up but macro-infested wreck in the waters just beyond Khao Lak ... but apart from just a couple of wrecks at trimix depths (>65 meters, I think) the tech sites out west are mostly just deep reefs and there ain't much other wreck diving to be had. Do your deep diving in the Gulf.
Hope that helps.
Cheers,
M
Each coast has a lot to offer; too bad that Oct. - Nov. is the diciest time on either coast. Good luck getting an itinerary sorted out, and please post a trip report after your visit to the land of smiles.
