I've had my Timbuk2 messenger bag since 2000 and it's still being used every day. Back then I used it simply as a book bag, lugging textbooks and the occasional 24 returnables. The waterproof lining means even in a downpour your books and/or laptop stay dry--just don't toss it in a river. Since college I've started using it on my bike and the cross-strap helps hold it in place. Just make sure you cinch the main strap as much as possible. I also now use it everyday for work as I find standard corporate Dell laptop bags to be oh so very lame. I have what at the time was size "Deedog" which I think now would be between a medium and a Large. I am able to fit a weekend worth of stuff in this bag, or for an alleycat a 12 pack, U-lock, blinky lights, tools, shirt, snack foods, markers, etc etc, so on and so forth. If you're on the fence get the bigger of the two sizes you're looking at--you don't need a man purse you need a messbag.
These bags start out stiff with that waterproof lining, but with use and age they soften up and conform to your back. I have very few items in my house still around that existed in 2000, and even fewer that are used daily.
There are plenty of messbags out there with more bells and whistles, but you can't beat Timbuk2 for straight up quality vs affordability. I repeat MY BAG IS OVER 10 YEARS OLD AND STILL KICKIN IT.