The prepaids are all pretty much similar.  I used 12call(AIS) when I was there.  For calls I really would suggest using IDD call cards.  They are just as easy to get as prepaids. You can even register the IDD card to your mobile number, just like iTalk.  So you only need to register once, call the number and you'll just have to hit the number you want to dial. 

Using pre-paid to make overseas calls.. well, you would be reloading quite a bit then, so you might as well use an IDD card to save the troubles.  But if you insist.. then well, I think whatever provider you get from wouldn't matter if you can afford to fork that much.