I have the real Maxpedition Fatby Jumbo as well, use it as both a camera bag and EDC. The material on the Maxpedition looks more rigid, which is a plus since I use the MOLLE a lot for extra pouches in which I carry stuff like lenses (which weigh a fair bit).
The most noticeable differences are the mesh pouch (which is a collapsible Nalgene water bottle pouch on the Maxpedition, which also holds a Thermos coffee mug or a 70-200 f/2.8 lens quite nicely), and the absence of the cellphone pouch (there's a row of MOLLE over a large ALICE style strap; I attached a Maxpedition M-2 waistpack to mine for a cellphone and my IPAQ 2410 pocket pc/pda). On the new Maxpedition versions, the MOLLE on top of the top pouch has also been replaced by velcro (i'd rather have more MOLLE real estate).
The shoulder pad on the Maxpedition is very comfortable, even when carrying a heavy load. These bags (both versions) are a bit deceptive in that they look pretty compact but hold a surprising among of stuff. The ability to mount extra pouches makes it unbeatable.
This is now pretty much the only bag I use for photography, including on stage at a concert last summer; it's comfortable, it's easy to sling out of the way, and everything stays easily and quickly accessible.
I'm curious (didn't bother looking it up) how much the Pantac one cost you; there's another non-Pantac ACM version on ebay, which appears identical, and goes for really cheap. If you aren't carrying a heavy load, or subjecting the bag to much abuse, I think the knockoffs would do quite well, for a fraction of the cost.