No offense meant, but do you really want to give the Burmese government your money? The diving in the Similans seems just as good.
No offense here too Arizona but the diving in the Similans and the diving in the Mergui Archipelago are very-very different.
I have dived both the Similans and the Mergui Archipelago extensively and dare say that
the Mergui Archipelago has a much richer variety of invertebrates than you can find in the Similan islands. Another big difference is the "color" of Mergui; there's lots of coralline algae growing on the submerged limestone rocks and giving those rocks at many of the divesites a very distinct color.
And then there seems to often be a
better variety of fish-life as well and often
more fish than on sites in the Similans too.
The Similans tends to have the better visibility and some amazing granite boulder sea-scapes that Mergui misses. (although North and South Twin do have granite boulders as well).
Mergui still has some ongoing problems with destructive dynamite fishing and the Similans have the ongoing problem with general overfishing from Thais fishing trawlers.
Lastly I like to say that in Mergui one will really get the feeling "to be away from it all" and that in the Similan Islands this is nowadays not the case any more. There are nearly always other boats with divers at any of the sites that you will dive and at the most popular sites there may be up to 12 or more boats at one time.
Still, both areas offer some very good diving and in my opinion a "perfect" itinerary would include both the Similans (and Richelieu Rock) and the Mergui Archipelago.
Cheers from Cebu!