Tough question. I'm replying because we visited both in 2012. I think I failed to find the most attractive part of Fougeres (so do careful map study), and it was very quiet the day we went, maybe it was a Sunday or a holiday the week before 14 Juillet. It was also way off the main highway, took forever to get there. I was very satisfied with Vitre, although it could be characterized as touristy - but not to the extent of Dinan (which has some commonalities ... ). OTOH, there were nice places to eat, and with some driving around, we found affordable hourly parking. I wish we'd had time for Mme. Sevigny's mansion outside Vitre. On balance, I vote for Vitre. But you should look at Google Images, too.