I have BOTH the backpack AND the Messenger, and my girlfriend has the sling, so I think this qualifies me somewhat as an expert...
In short, the backpack is great! It holds an enormous amount, and was all I took with me on a recent weekend trip to Boston (2 days, and I'm a BIG guy, so I have BIG clothes)!
Charging system in all the bags is wonderful, as long as you are not planning anything but a smartphone or iPad (Won't charge my ASUS Transformer TF1-01).
Messenger is a waste of money. Due to the placement of the battery, it is all but useless for anything but a tablet, Mac Air (won't hold my MacBook), and a THIN notebook or newspaper. Foget lunch - it's not big enough for a sandwich and apple when fully loaded. Outside "water bottle" pockets are a joke!
Sling is somewhere between the two. Also doesn't hold much, but it isn't intended to; a light jacket, tablet, phone, wallet and umbrella is about it. My GF uses hers as a casual, medium purse.
If the backpack is a 10 and the messenger is a 5, the sling is about an 8.