Based on my anecdotal evidence (relatives shuttling between Bangkok and Chiang Mai), and my own experience earlier this year, I'd say that Nok Air seems to be on schedule more than TG. I can't recall a Bangkok-Chiang Mai-Bangkok flight on Thai Airways in the past 5 years that I've taken that hasn't been delayed. I can't comment on the Udon Thani flight, though, as I've never taken it.