You can't go wrong either way.
Chiang Mai is a fantastic travel and vacation destination with great restaurants, bars, shopping, and spas. The town has a nice style, and the best part of being there is wandering around and discovering it on your own. There are nice cultural spots too, including a few extra worthwhile temples to explore. It is one of my favorite places.
Having said that, if I were you, I'd seriously consider the side trip to Siem Reap for a few days at Angkor Wat, because Angkor Wat is one of those "bucket list I'm so glad I saw this before I died" destinations. The town of Siem Reap has come a long way as well, and you'll find enough interesting things to keep you busy for a few days.
Just remember that when you return to Thailand overland, you'll only get 15 days visa exemption at the border. But from your other post, it seems like those 15 days will be enough for you to complete the rest of your frenetic itinerary. I hope that helps. Cheers.
Chiang Mai is a way cheaper option and definitely worthwhile if you do some research and get a bit off the beaten path. Flights are cheap bought in advance. Flights to Siem Reap are ridiculously expensive. Oh I live in CM so go figure ;-)