Hey Ash, I've no experience with the others so i'm afraid so can't help you much with the other sacks. But I have recently bought the LA Salient and so far have not regretted it, you may have seen my thread where i was torn between it and the sabre 75.
(Granted i've only had it a few days) but have filled it up a lot to try it out and will be going out properly with it this weekend. it feels very well made. only problem ive thought of so far is water resistance so will probably be buying a raincover for it soon.
I think i'm right in saying that it is the smallest capacity of the bags you listed, and it is quite a tall bag when filled up, especially when the compression straps are tightened. this probably makes it more important to get the fit and weight distribution right as opposed to fatter bags because of the possibility of it tipping one way and the other, but the adjustable back system helps with that, and once i'd I pulled the straps tighter to my shoulders it seems to carry well.
Plenty of molle straps so no problem with attaching extras on the outside, but I suppose it comes down to how much kit you want to carry in the body of the bag as to whether 70l is enough. BUt, with floating lid, 2x 10l side pockets, and possibly a 20l pack on the front that will all add up. AND FINALLY, one thing that always put me off about adding side pockets was how wide and awkward the pack becomes, but that would be much less of a problem with the salient because of the skinny pack and smaller pouches.
sorry i wrote so much, hope at least some of that helped!