Chiang Mai or Koh Samui

Hi All,

I will be traveling to Bangkok in November. I plan on also visiting another city in Thailand, and I am debating on whether I should go to Chiang Mai or Koh Samui. In Thailand, I have only been to Bangkok and Phuket.
Any input is greatly appreciated.

I will be in Thailand for a total of 7 days, and I wanted to split Bangkok with another city.

Thanks in advance for your time.