Bejay,
I don't know if we ever got the chance to discuss this in detail before I left town. But I would highly recommend going with the larger size. Some reasons:
1. Since its a backpack, you won't have to really worry about the bag sliding around your back when its minimally loaded (as compared to a "single-strap" bag).
2. The more packages (and remember, these can be anything, such as a 8"x8"x8" box AND 3 blueprint rolls AND 2 garment bags) you can carry, the more money you'll make, as well as get on the good side of your dispatcher when you tell him that you'll take the large pickups without hesitation.
3. That box that someone is riding with on their handlebars only slows them down. If you could fit it in a larger bag, which you can then carry on your back, then you'll be faster (and indirectly make more money).
Sure, the large backpack is a bit cumbersome when you're getting a beer after work, or anyplace where you need to squeeze between people. But if this is truly your work bag, you'll want the largest size possible.
Lastly, when you order whichever bag, request an extra "External Side Pocket". Trust me, it'll be worth the extra $10 (or whatever).