Why not splurge and go to Micronesia? Or French Polynesia? That's world class diving but not cheap!!! Cheaper and also interesting is Bunaken out of Mindano, Sulawesi, Indonesia. Or if you are doing Philippines go to Donsol south of Manila and to the east. Supposed to be the whaleshark capital of the world..... or so they say.....around april/may is the best time.

A colleague of mine from the Philippines who I worked with in Thailand owned dive shops in both places (not at the same time). He said he preferred Thailand. He claimed there was less big stuff in the Philippines. But it depends where you go in the Philippines and when. Also, in the Similans now there is a lot of noticeable damage to the coral due to the tsunami. Still is beautiful though. Was in Khao Lak when the tsunami hit and our house was destroyed with us in it. Barely escaped alive from the huge wave smashing thru our house carrying motorcycles, fridges, you name it along with it. Had to punch our way thru the roof to get out alive, away from the deadly river raging thru what 30 seconds before was our room. Water so powerful all the clothes were ripped from us. The entire village was a mess. Last time I was in Khao Lak the place was still pretty scarred up but they were rebuilding and the bad parts of the reefs in Similans can be avoided. So you'll still find good diving there.

You could also go to Sipadan on Borneo, Malaysia. If you got 3 weeks and it's your only trip of the year........SPLURGE.....and don't forget to take me.