I say stay in St. Malo. It's very much a holiday town (mostly British visitors, very few Americans in my experience) and there are plenty of hotels &nice places to eat. The walk around the city walls is great, and the beaches are good, too (if a bit rocky...it's the Atlantic, after all!)

Several years ago I spent a night in St. Malo. Next morning I easily made a local connection to Rennes then the transfer to Paris...didn't see any of Rennes except the train station. I made my hotel reservation in St. Malo after getting off a train from somewhere else, and would actually recommend doing it a bit sooner (use a good guidebook or the net - go for something in the historic center, although it will be more expensive.)

One thing to remember is that St. Malo is far west in its time zone, so you will have LOTS of daylight to see the town. Even in early September, when I was there, there was light until well after 8:30 at night, and I felt perfectly safe walking around.

I recommend the cathedral. It was heavily damaged in WWII, and the restorations are fascinating.

Julie in Seattle