I would say for a really light load, messenger would be best. If you get a decent one, it will have a sternum strap to prevent it from swinging round into your lap throwing you off balance, and will also lay high enough up on your back to take what little weight there is off your shoulder. It can be a little bit scary carrying like a soup or other liquid meal in a messenger bag as it may not sit horizontal to the ground and be susceptible to spillage (though I haven't experienced this.

Also, no one has mentioned this, but with a messenger bag, you can wear it over your right shoulder (or have it made for wearing this way) and will have no blind spot when looking over your left shoulder in traffic (if you live in a right hand drive country); with a back pack you will have a blind spot if you are riding on the hoods or especially if you are in the drops.

Finally, some messenger bags come it with a removable 2nd strap that can go over your other shoulder when carrying extra heavy loads. Check out Bagaboo in Hungary. Incredible indestructible bags. Have the 2nd strap. Usually I carry a light load, but carried a 30 pack of beer with the 2nd strap and it felt fine. Also it has a U-lock holster on the outside which is really convenient. Keeps the dirty lock out of the bag, and is easy to access for quick locking. I got the large, but wish I had the medium.

Anyhow, I would try to borrow both a backpack and a mess bag to try out. I'm very particular about my bag and how it rides, and after lots of attempts I think I've found the right one. Good luck.