This is a great question, and unfortunately I don't have the answer because I have not been to Belize or Roatan.
Let me ask what you plan to do in port? With your new knee I tend to doubt you will want to snorkel or swim with the dolphins. It sounds like you want to take it easy.
I do not think Cozumel will be overrun with drunk college kids (they are probably thinking of Cancun which is on the mainland). It has its own port that offers shopping that the locals cannot reach, so you can get off there and look at the typical souvenirs you see any place in Mexico. I would not bother going into the main town of Cozumel unless you have never seen Mexico before - it is a typical Mexican town (nothing special).
Other than that, there are beaches, horseback riding, scuba diving and other things on Cozumel.
But I think you need to tell us what you would like to do, and can do physically, in port - if you can swim, snorkel, etc, before we can give more advice.