Personally I don't think any of the "tourist" SIM cards are a good deal. I like going to the FamilyMart just past all the AIS/True/dtac counters and getting an 88THB AIS 4G Net SIM (1mbps for 7 days) or Zeed SIM (500mb for 30 days) for 50THB. Depends on what your needs are. Can then top up or change packages via the AIS app or I believe using vouchers and SMS codes. The only thing you can't do, I found out the hard way, is add a SIM2Fly package, those apparently can only be added to SIM2Fly SIMs (which I think can also be topped up and have a local plan added, but I've never tried this).
The FamilyMart also has regular SIMs for dtac, I don't remember if True was there. They will also register for you. Having done this twice, it's as easy as the telco counters and much less expensive.