I use both for different purposes. I have two bags, both from R.E.Load, which is a local company that makes some very nice bags. My messenger bag is 5 years old and still in great shape. It's good for quick trips and light loads, or when I need easy access to my lock. That said, it hurts my shoulder when I use it for too long. It also slips, which drives me CRAZY. No matter how tight I make the strap, the bag will drift to the right side of my back when there's anything at all in the bag. I find that I have to shove the bag back up onto my back every few blocks. Annoying.

The backpack (called the Midpack) is amazing for commuting and one-stop shopping. It's 100% waterproof and made from cordura, just like the messenger bag, but it's much more comfortable. It doesn't slip at all, and thanks to RELoad's bike-centric design, I can adjust the straps with one hand. It also has side pockets that are perfect for a lock and a water bottle. It does make my back VERY sweaty, however, so for summer riding I'm looking forward to some new panniers for my daily commute.

More info about and pictures of my bags can be found here, on my blog.