We'll be leaving Mont-St-Michel on a summer weekday morning to drive to our CDG hotel to stay the night before an early morning flight. Do we have time to stop in Dinan or Chartres for a few hours?

Well, you can check it on www.michelin.com or www.mappy.com, using the exact route you'll be taking, but count on about 4 hours' driving, so yes you can make a stop.

Chartres would be a better stop on the way back to Paris; Dinan is a bit out of the way.

After seeing photos of both, it's hard to choose just one (but leaning to Dinan for it's medieval beauty seen in photos; and since we will see other cathedrals in Europe, although perhaps they don't compare with Chartres). Dare we try to do both; opinions on one or the other from those who have been there?

I suggest a different thing.

A route via Fougères, Vitré, Laval, Le Mans, Chartres, Paris. This entails leaving out Dinan (a place I know and like a lot) and substituting an even more interesting medieval city in Vitré. See http://www.ot-vitre.fr/pays/merveilles.html

Agree completely with Padraig. Fougères and Vitré are more interesting than Dinan, especially if it's medieval architecture that interests you. Nice route.