Local SIM gives you reasonably fast internet everywhere... on the street, in a bar, at massage, etc... Don't want to miss that Line message from your next ThaiFriendly lady! Cheap enough. But, if you need to receive/make calls from your home number, then international roaming is what you'll want. If your phone does dual SIM, then both local and roaming coexist. I think AIS now supports e-SIMs, so a modern iPhone can have local (e-SIM) and roaming (physical SIM.)
Local SIM and tourist package can be purchased just outside customs at Suvarnabhumi at carrier kiosks, at carrier stores in Terminal 21, or at many convenience stores. Study up on the various packages available. Clerk will swap out the existing SIM in your phone for the local one and activate it, then pack up your old SIM so you don't lose it. You'll need to show your passport. For iPhones, you'll also need to enter your Apple ID password to finish activation.
Probably best to buy a cheap package and top up occasionally to keep the number alive and add more data.
https://www.ais.co.th/travellersim/
Top-up: 39 baht for 8 more days and 3 more GB
