if you stay only in Bangkok, get pre payed 3G sim from True Move, or AIS, both have pre payed packages cost 599 baht for 30 days of 3G,

you are connected at 3G speed until you have reached the 2Gb download limit, after that your download speed goes down to about 128 kbs, enough to receive and read email etc.
For 799 baht you get 3G up to 5Gb of usage, both True and AIS pretty much mach the prices of their 3G packages.

In Pattaya True H has been not very good at all, past few weeks, some days you are disconnected for maybe 30 minutes or so. But if you restart the phone, it connects with tower, and you are connected again. Or put in Flight mode for few seconds, and then off flight mode.\

AIS is much more stable in Pattaya.

Before BrisbaneGuy asks about research cost, I know about 3G coverage because I use both True and AIS in Bangkok, and in Pattaya.