Pattaya - ultra touristy, plenty of sleaze, poor sea, issan's home from home, not really like anywhere else in Thailand. An easy introduction to Thailand for foreigners, closest likeness, a cross between Blackpool, Morecambe and Clacton. Wheels not necessary.
Chiang Mai - great scenery, more normal Thailand, a bit reserved and snooty in places, heavily polluted at certain times, beg packers delight, frowns on Issan. Closest likeness, a cross between Skipton, Harrogate and ? You need wheels.
Bangkok - Phew, I couldn't even begin to describe or compare.