It depends entirely on what you use it for.
messenger bags suck when you back is relatively verticle because they put weight on only one shoulder. When you back is relatively horizontal most backpacks are really unstable though.
-If you ride in a really upright position definetly get a backback
-If you spend alot of time off your bike definetly get a backpack
-If you don't do either of the above consider a messenger bag
-if you want to look cool get a messenger bag.