Pai or chiang rai better?
They are different.
Pai is a small mountain town that's popular with ang moh tourists, and is one of the small towns that is part of the famous 400km Mae Hong Son loop (popular with motorcyclist). Around Pai are the mountains of Mae Hong Song province, so you'll have the usual mountain attractions. I was there (for the motorcycle loop) and it is a touristy town with a nice vibe, like a small, mountain version of Luang Prabang or Hoi An.
Chiang Rai is a city that can be used as a base for exploring the Golden Triangle, and itself has a few attractions like the White temple, the Black house. It's bigger than Pai, and also has bigger night markets, including a really massive one on Sat night.
If you are into mountains or you want to explore mountain roads on a motorcycle/scooter, then Pai is good. Else, Chiang Rai is more suited for most people.