My sister and I did the touristy thing and compared Fairmont vs. St. Viateur. Obviously, if you're in the area and have the time, just go to both! They're about a 10 min walk from each other. Don't take my (or anyone's) word for either- decide yourself!
I personally preferred St. Viateur over Fairmont. I just stuck to the sesame bagels and cream cheese, and thought that the bagel was a little too sweet for my liking. My sister got a chocolate chip bagel which... Just don't. Stick to the sesame. She said it was hard and not fresh.
3/5 from the both of us. But like I said, everybody's taste is different so go and try them out yourself!