I've been using a Camelbak blowfish for rides around the city (never more than ~50 miles roundtrip), and I love it. It has decent storage capacity (could pack a pump, small toolkit, a layer and some snackies) and then unzips to expand. The back padding is designed to encourage airflow.
I was also going to mention the Deuter (though I have never used one), as I've heard good things about them and their suspension system seems clever. (Although I've heard from some that when combined with the hydration system, and when the reservoir is full, the pack bulges out a bit and keeps the suspension system from working as well. Still...
I might discourage the messenger bag. As someone above mentioned, they're as much about gaining ready access to the contents as they are about carrying the goods. If you don't need that quick (and repetitive) access, a traditional backpack distributes weight better and stays put. (I'm sure some will disagree, but I found messenger bags had a tendency to shift and that it required the occassional repositioning push with the elbow)