Thanks for you post. Yeah the 50L looks really nice. I'am having trouble finding a retailer that delivers to Canada though.
I'd stick with the Arc'teryx product. The Predator is predominantly designed as a daysack / patrol pack for wearing over body armour and lugging all the stuff around that a modern infantyman needs out on patrol in somewhere dusty and sandy.
Again you get just 1 back length which is fine if it fits. Also the waist belt is there but it's not particularly padded. If the back length is incorrect it'll end up around your stomach not sitting on your hips so weight transfer of the load onto your hips will be compromised. Ok for light loads but if you load it up with 50 l of kit and then add another 20l in side pockets you end up with a 70l load being mainly bourne on your shoulders...
The Zip off main compartment is interesting but unless you are planning on lugging a PRC 77 Bowman or Clansman Radio or ECM kit around it might be a tad pointless.
The up side is the pack is well made and will last a lot of abuse and has a few 'real world' features that you'll use.
The Arc'teryx is not designed for combat conditions comes in a couple of back lengths so you get a basic option for it fitting correctly and thuis transfering weight to your hips correctly and I suppose the upside is being in Canada you can try one on before you buy it..
Arc'teryx has an exceptionally good reputation for innovation and quality of product I dont think you need to worry on that front...