I personally preferred Vannes, mainly because it's on the bay, has a great medieval center, and is near many pre-history stone monument sights. I skipped Quimper because I thought it looked a little bigger than my taste, and Concerneau seemed a little less beautiful than Vannes. I highly recommend Locronan, near Quimper, for a short visit.
In the Morlaix area, I visited Saint-Thegonnec, Guimiliau, and Lampau-Guimiliau. I liked these little towns but hesitate to recommend them because the area is quite sleepy. If you can time your visit with a Pardon (local festivals that run all over Brittany from late Spring through early Fall), I think you would enjoy anywhere in the area more.
If you do stay in Vannes. I really liked Auray for a short stop. I also love Cairn du Gavrinis, it's fairly recently discovered and only available as a ferry tour (in French only) from Lamor-Baden but a real non-touristy treat for anyone who is fascinated with the pre-history of the area.
Another place worth a stop IMO is Ft. La Latte on the north coast. It's an old castle that was repurposed many times over the centuries but still retains much of its charm and makes a great visit.
