CC, to be perfectly honest, I'm just getting learned on this whole pannier/commuting thing myself. As such, I am far from the expert on panniers and optimal volume for this or that.
As point of clarification, I don't think the Appalachians can be broken apart. Rather, I think that the two bags are connected by the fabric that sits on top of the rack. In that sense, they are pure saddle bags, a single unit which straddles the rack, with a bag sitting on either side.
As for what can be carried. In the 2 Cartiers, here's what I carry:
#1, jeans (remember, I'm a big guy, so these aren't small...
) , skivies, socks, undershirt, overshirt (polo, etc.), locker lock (eg, Master Lock)
#2, regular towel, shampoo, soap, deodorant, ulock, shackle.
Generally speaking, there is plenty of room, and I am not coming anywhere near "stuffing" the bags. I'm sure there is more I could get in there if I really wanted/needed to.
If you need the bags for commuting (eg, need to have a change of clothes and grooming supplies), the Cartiers give you a nice amount of space, and some overage, in case it's ever needed. If you need it for around town hauling, and carrying, then the Appalachians might do you just fine. I think that I ultimately steered away from the Appalachians because you cannot separate them. Meaning, if you ever wanted to put a bigger bag on one side, or a grocery bag on one side and a normal bag on ther other, you were out of luck.
Ok, my 2 cents... or 99
Regards,
Sam