During our first trip to Thailand in 2012, we did a 2 day / 3 day split between CR and CM, flying into CR from BKK and then busing it down to CM. We started off in CR to allow for a more easier trip to the Golden Triangle and tour the regional sites on one day. I'm glad we visited CR but it is a fairly small town. It does have a nice market area that we enjoyed. Two full days felt enough for us and I don't feel a pressing need to "force" a visit to CR but would like to go there again if circumstances align and be more adventurous touring the region.
Love CM. We've gone back twice since our initial trip and I'd love to go again. It's bigger and livelier. Really enjoy its various market days/walking streets but they do get packed. There are a number of things to see and do in and around the city as well as daytrip activities outside of town. Be cognizant of some of the more kitschy and sketchy tours though.