My suggestion is to go to REI (as suggested above) if one is convenient. You can try various bags as they will weight it down for you so you can assess comfort. One thing I will warn you about regarding roll top bags is that SOME can impede your ability to look backward, even with a helmet or eye glass mirror.

BTW...I own and use back packs and messenger bags. Messenger bags, FOR ME, are somewhat uncomfortable off the bike, but very comfortable while riding a road bike. I've never tried either with my hybrid (upright positioning) because it has a rack which I utilize. The quality of the backpack or messenger bag is also crucial. Osprey makes great backpacks. I tried a Timbuk 2 messenger bag and didn't like riding with it. It's now my bombproof attaché bag for work. I use a Chrome messenger bag which is great for me.