There aren't any really hilly bits whichever way you go, the route along the canal between St Malo - Rennes is nice (and dead flat), other than that it's your typical rolling French countryside of cereal crops & sunflowers changing to vinyards as you get near the Loire with plenty of small quiet lanes to choose from. Personally, I'd pick a route from St. Malo to the river around Angers then follow the Loire cycle route to Tours. There's no shortage of campsites and more chateaus than you can point a bagette at.