My family moved to Koh Samui last, year we have small kids, a big dog and small dog.
We moved to Phang Nga recently only for the sake of staying in Thailand.
We were extremely happy on Koh samui. It is paradise.
Chiang Mai is also a wonderful place but I wouldn't want to live there during the burning season so if you plan to go there I'd also plan to leave during that time of the year.
I also want to live in Chiang Mai in the future but only briefly.
I plan to live on Samui again in the future and possibly settle there more long term.