I sold rucksacks at Blacks and the scout shop for years.
and i have 20 plus years outdoors experience.
in my opinion civilian rucksacks will without any doubt Always be more comfortable and will have the mod-cons than surplus sacks(or copies).
Surplus versions (if new) will be very tough! they may outlast you! but will be less comfortable.
the danger with huge sacks 80l+ is that you will want to fill them, and military ones tend to have space to add even more on the outside.
my best advice is try loads of different sorts, Berghaus and Karrimor are a good start point, It's worth noting that PLCE Bergens took it's design from Berghaus rucksacks(like the Vulcan)! it's also worth trying some dirt cheap ones just so you can see the difference, (usually fabric and stitching quality)!
we used to sell "cheap" rucksacks at the shops for £60, the sales took them down to £30, then £15 and guess what we were still making a profit!...
some fell apart and were bought back and it cost more to get them fixed than the bags actually cost! so we cut off the buckles and sold them instead!
which do i use for the last 15 years or so? (I've tried them all)
Olive PLCE bergen, and Large US ALICE(with frame).
PLCE in winter (more waterproof but it gives you a hot back)
ALICE in summer (loads of air flow but not as waterproof)
when it comes down to it, it's your choice, but try as many as you can!