Re: J34 or J107?
I have the J107 and really like the pack, but if I were to do it again I would get the J34, Its a little bit better suited for the 3-4 days hikes and is nearly as big. It can be used as a day pack as well, where the 107 is just too big.
I also find the J107 a little bit heavier than I would like.
*forgot to add* if your planning on using big optics and/or a bi-pod, get the M version of either, as the scabbard is much better suited for LR rifles.