I have the larger J107, but have never used the J34. What I can say about mine is that it seems ultra-durable, capable of loading all your gear, with enough seperate compartments and configurations to fit multiple situations, and is quiet. On the negative side, I have packed a few heavy loads of meat now, including an entire boned whitetail, most of a very large boned muley, and most of a boned elk calf and while there is tons of room, the bag definetely "hangs" under the heavy load, and does not stay close to my back. The straps are great and are easily adjustable, but it seems to be in the frame. Clearly I may need to adjust the frame, but to date have not found what I consider a satisfactory fit.
I am sure you have been to their website, as they have a fairly clear comparison of the two models. They appear to be almost identical, with the J34 just about 20% smaller.