The tarp-like inside surface is a PVC tarp liner sewn into the bag and not bonded to the Cordura. With the standard bag, there's no choice but to line the outer pockets, flap for the outer pockets and the main compartment with a sturdy PVC lining to make the bag weather resistant. With the AP bag, the outer pockets don't have a PVC liner because the PTFE membrane bonded to the Cordura will keep things dry although you will notice the flap for the outer pockets still has a PVC layer sewn in. The main compartment also has a PVC liner but it's much thinner than the one on a standard bag because the PTFE aids in the weather resistance so a thicker liner is not required. It's nice that the liner isn't as stiff on the AP bags but I find the VX fabric to be much stiffer than standard Cordura. With a bit of use, it does loosen up somewhat. I do worry that the thinner liner might be susceptible to damage from hard pointy objects compared to the sturdier liner of the standard bags. Of course, I have a friend who has a standard Monty and the PVC is cracking on the flap for the outer pockets because she stuff them far too full and it causes the flap to sit awkwardly.
The MW is one of the nicest looking bags out there. The rectangular design makes it ideal for carrying books, binders, files, laptops, etc. The pockets are very nice for commuters. The roll-top is great for really keeping stuff dry. I find a lot of messenger bags to be well shaped for aerodynamics on a bike but their shapes tend to make it less ideal for fitting bulky items or items that are hard with no give. But the bag isn't perfect. I find that accessing a fairly full main compartment even with the strap fully expanded to be an exercise in futility because of the tall walls of the roll-top closure. The depth is 5.5 inches which is a little narrow for carrying bulky items so you tend to fill the bag vertically which means whatever you're looking for invariably ends up buried under too much crap. I don't find it's a good "working" messenger bag. If I have to access stuff frequently, I keep them in the outer pockets. I'm considering getting MW to custom build a longer strap so the bag will sit lower when I expand the strap fully. It's comfortable but I feel the PAC is more comfortable especially with the removable back pad to keep the pointy things from jabbing my back.
So the AP versions have nicer inner linings, thinner fabric, and may even be lighter than the standard? Arghhhh why did I discover this brand before they started selling the AP iterations.
You hit the nail right on the head with the "tall walls" comment--I keep some things in the bag's zip compartment for quicker access.