I would like to visit one of these two places after Chiang Mai traveling solo.
I keep hearing about Pai being too "touristy". Would I be better off going straight to Sappong?
Any recommendations on where to stay?
How busy will these two places be during the last two weeks of May?
Pai has become a joke. Nothing more than dippy/plastic hippie hell. Stopped visiting there years ago when the first 7-11 appeared. After that things have only continued to get worse. Could not pay me enough to spend another night there. The moment the herds of sheep appeared prices soared and the typical scams started. You can thank the LP Guide book for that.

Soppong over Pai any time - by miles!
Agree with #1 about River Inn - magical place, especially if you get one of the rooms that hang over the river edge. Really good food too, and a base for local treks. Highly recommended from a half dozen stays there.

Depends what you want to do. Neither Pai or Soppong are that good. Lot cave is very good but apart from that there's some hot spring baths in Pai and little else. Pai isn't that expensive as Tit makes out. Pretty much similar to the rest of Thailand that receives tourists.
May (after Songkran until mid-June) is usually pretty slow tourismwise so Pai will be a lot quieter than, say, Nov-Feb when Bangkok tourists pretty much take the place over by the bus and vanload. As for the backpacker scene it can seem pretty incestuous and repetive if you just hang around the same restaurants and bars in town, sort of a mini Khao San Road social phenomenon in the hills. The good part is the hills themselves, so get the * outta town on a bicycle or small scooter and go exploring. Then it's just another 45km to Soppong which is really peaceful and much quieter with lots of good DIY walking trails and other outdoorsy stuff, or just plop in a hammock with a book. Why not visit both and see for yourself?