The only difference (besides the capacity) I could find was the side mesh bottle holders: the 65 has two separate ones while the 50 mesh holders are integral parts of the larger wet-pocket. This might be an issue when the pack is fully filled: you may not be able to fit bottles into the mesh on the 50. You're less likely able to get your bottle in/out while hiking with the 50, more likely with the 65. I'm not really a bladder-tube-feeder, so bottle access is a more important feature.
I'm able to pack the 50 with my gear and a bear canister placed vertically at the top. The 65 easily holds the canister horizontally and has some room to spare. My weight measurements indicate a 1/2 pound difference between the backpacks.
I've seen plenty of posts about using a 65 on a thru hike, but none for the 50. Is the 50 is sufficient size for a thru hike? I know the UL answer (sure it is), but has anyone used it on a thru?
Thanks for any input. I'm enjoying all the info on this board. I follow the new posts daily now. I'm working up a Tahoe Rim Trail hike with some other old guys....JMT is further out.
Doug