I'm a bit jaundiced about Chiang Mai - it's changed from the rather laid-back place i remember from years' back. On the plus side: the CM folk are, in general, less focussed on getting into your wallet than their Bangkok counterparts; the old town has charm, and temples aplenty - but the big-name ones can get horribly over-run by camera-clicking tour groups. There's street food and some interesting restaurants: and there's breath-taking beauty in the mountains around, but it's not so easy to access.

And you have to remember CM is no village. It's Thailand's second city. It's BUSY, and packed with tour agencies/"tourism advice" offices all retailing the the same set of tours organised by the Big Boys. There are quality, independent operators, but in reality they are visiting (mostly) the same spots as the others. If you go, I'd suggest renting a car and/or hiring an independent guide.

As for Siem Reap, the town maintains a measure charm, but it's tourism focus is pretty much on Angkor, which can be spectacular. The temple/archaeological complex(es) are vast and spread out over a large area so a canny guide can keep you separated from the hordes. If you are into exploring the sites, this could take up a few days.

Their are some really top-rate hotels there, and a host of less expensive "boutique" establishments as well as the cheap and cheerful. Prices for quality names can be absurdly high, so tripadvisor and something like trivago are probably good friends to help sift through what's available. You should aim to stay close to the town centre: La Residence or Raffles are expensive options, the FCC cheaper and quirkier - but there are other places to look into: once you start a "which hotel in SR" discussion here, it will go on and on ....