"Sorry Tom. Having owned an Ohm and now a Porter, the Ohm has proven to me not nearly as durable off trail. The proprietary fabric used by HMG is much more abrasion resistant as well as tear resistant. The cordage used on the Ohm will catch every branch. No such issues with the Porter. The frame is stiffer on the Porter as well. The Ohm is a fine pack but not what I would consider a bombproof lightweight pack, which the Porter has proven."
I guess I'd chalk it up to different environments, David, or maybe a different way of moving thru rough terrain. I have had no durability problems with my OHM's whatsoever in 4 years of hard use, other than one small tear in the kangaroo pocket on my first one. One environmental difference that occurs to me is that the Sierra is much more open than I imagine your Canadian arboreal terrain to be, although I have not had any issues with cordage(compression lines) hanging up on branches or fabric tearing other than the one I mentioned above, either in the Sierra or the Cascades when I have encountered that type of terrain. That said, I have no way of comparing, since I have not owned a Porter, but the OHM has proven plenty durable enough for any packing conditions I have encountered. If I were doing most of my packing in very rainy conditions, or wanted to carry loads over, say, 27-28 pounds, I would definitely consider a Porter. But for my typical load and climate, the OHM 2.0, which is $60 cheaper and a minimum of 3 oz lighter if you don't include side/hipbelt pockets, is a better deal.