The suspension is exactly the same on both bags, so they should top out at the same max weight.
Looking side-by-side at them in store, the difference between the 50 and 70 are solely volume. The 70 is deeper (from frame-sheet outward), and taller. It could possibly be wider from side to side, but its hard to tell with so little structure to the pack.
I tried them both with bean bags and they both got uncomfortable fast at >40lb. The 70 had a good bit of room to shift the weight around, the 50 was stuffed full. I'd say 35-ish is the max load for comfort.
In the end, I bought the 70, to take it home and test-fit it with my gear. I'm still on the fence whether its overkill or not. I can pretty easily fit all my gear in and the pack is below shoulder height, but it looks like a beach ball on my back. I can get it to stack much taller by limiting volume with the straps, but its a tetris game to get it all in and balanced.