Can't decide between night at Mont St. Michel or St. Malo... thoughts?

I love both MSM and St. Malo... I did spend the night on MSM and have to say that while there is not a lot to 'do', just the experience of having the place virtually to yourself, wandering around, looking out over the water, is one of the most memorable nights I have had. I was there in Nov. with my 15 year old grandson and he still thinks of it as one of the greatest experiences. So... for us it was well worth it. Even though it was winter, there were still a great number of people there in the day,, so I can't imagine what it is like in the summer... must feel like Disney. So, for me, I would want my 'alone time' there and stay on the island. St. Malo was great for a few hours, we ate, shopped and wandered around, but I did not regret having time to spend the night there.

The RS book suggests arriving at MSM in late afternoon to miss the crowds, staying the nite on MSM or nearby, and then another visit in the morning if you want before crowds arrive. We did this, and it worked out great! We stayed in one of the hotels just across the bridge so MSM was just a shuttle-bus ride away. And at nite the lit-up view is incredible.

So I'd suggest driving to St. Malo , spend a few hours there if you want. I thought 2 hours was enough for me, you walk the ramparts and then thru town. But if you poke into shops and have lunch I could see it stretching to a longer time. And it sounds like you already have some ideas for the area. Also could take the ferry ride across the river and walk around Dinard, a nice beach town with some interesting architecture. Or visit some sights along the road back to MSM such as Pointe de Grouin or stop at Cancale for oysters, perhaps drive thru a bit of the Calvados countryside. I like to have more things planned out in advance than I can do and then adjust based on the weather, energy levels, etc.