Disclaimer: I have no backpacking field experience. Yet. This is my second backpack (not counting ordinary school backpacks). My first backpack was a 90L external frame backpack. I got this 50L because I wanted something a little smaller and the 90L wasn't terribly comfortable.
This thing is super comfortable on my back with around 30lbs of stuff in it. It fits me like a glove. The stitching looks good. No loose threads anywhere. However after packing, trying it on, unpacking, packing, and trying it on a handful of times at home, I doubt this thing will last long. I can already see areas where the seams want to split (the stitching is spreading out). With a lighter load, it would probably be just fine. Therefore, I'm thinking the 40L would be excellent at these prices ($40/40L, $50/50L) if you only need 40L. Again, nothing has ripped yet but I expect it would out on the trail for 3+ days.
Pros: Fit, feel, looks, price. Cons: Not super durable but not super flimsy either. The zippers on the belt pockets were a little jingly, so I ran some cord through them the way most of the other zippers have cords in them.