I have the Lowepro aw200 and now the Tamrac velocity 8x.

I think the aw200 can hold a little bit more than the 8x can, but I prefer the flatter profile of the 8x. The 8x also can hold my d300 with battery pack and a 70-300 (Tamrac says it's also no problem with a 70-200 f2.8) on it but the aw200 cannot. I now mostly use my 8x for that reason.

At the end of the day, a slingshot bag is still not a backpack in terms of weight distribution between your shoulders, but it's more convenient to use than a backpack (I suppose you have to take off your backpack to access your gears in it). And, if you don't have any problem with a shoulder bag, then I don't see how you should have any with a slingshot bag since, after all, you can carry the latter in the way you would do with the former. But when you're simply walking around, I think it's better to have the bag on my back.

Lastly and obviously, there's always a limit to the capacity of any bag