I rock them both.
My backpack:
+ bigger than my messenger for large loads (non heavy ones)
+ Lots of pockets for EVERY LITTLE THING in my apartment
+lots of straps (chest and waist) to keep it from shifting
- doesnt colapse as well as my messenger when i stuff it under my pedicab (or other tight place) for storage due to all the padding
- Heavy loads make me unstable with my backpack (center of weight is about 6" behind my lower back) even when the bag is hiked up high.

Chrome lg messenger
+ Most of the weight is distributed along my chest by the strap and not on my shoulder
+ most comfortable while riding (hiked up high on my back, oh yeah)
+ stays in place with out using the extra strap
+ easy to grab items while in motion
+ most comfortable when carrying heavy medium sized loads.
- cant carry as much as my backpack
- not as much padding as my backpack for wierd pokey shaped objects
- too few pockets (as compared to my backpack)

those are the + and - of my two bags. my backpack is a large bmx backpack similar to this one.
http://www.danscomp.com/730013.php?cat=STREETWEAR
but bigger.

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