Thnks all for the helpful advice.

At this point, I think we have decided to hold Mont St. Michel for another trip.

I know we will return to France, and considering they are reconstructing the pedestrian bridge and returining the site to its former glory (with water on all sides!), it might be worth the wait!

Here is a link with more info on the redevelopment:
http://www.projetmontsaintmichel.fr/en/barrage.asp

Link Paris offers a good semi-private mont st michel tour with rail.

http://www.linkparis.com/mont-st-michel.htm

I would choose Mont Saint Michel over the château country. Seeing one château is generally sufficient.