I've used VX-21 (what seems to usually be called Xpac), Gridstop, and 1000D ballistics in a number of projects. My thoughts:
VX-21's chief virtue is it's 200 psi waterproofness. I can't verify the number there, but do know that water does not go through the fabric no matter what. Tied to the front of my packraft getting blasted by waves, left outside and covered with wet snow, the fabric does not wet out, and gains very little weight at all. Obviously the seems still leak unless sealed. It's abrasion resistance is pretty good. I find that for bushwacking and as the bottom of a pack it holds up well. It's cut resistance is less good. I've had ski edges, sharp rocks, and carless packing of paddle shafts put small holes in it.
Gridstop has conventional waterproofness (it will wet through under the aforementioned conditions), and absorbs a modest amount of water weight under the same. It is surprisingly abrasion resistant, and very cut resistant.
Ballistics is fantastically abrasion and cut resistant, and will hold up to the most egregious rock draggin (canyoneering, hauling up offwidths) for an impressive length of time. The Seattle Fabrics stuff has a better poly coating than most packfabrics, and it's thus more waterproof than Gridstop, but not in the same league as XV-21. The face of the fabric does suck up a lot of water, especially once it's been fuzzy out a bit by use.
For me, VX-21 is a great default pack fabric. Gridstop is good for pack bottoms, backs, and side pockets which get a lot of wear. Ballistics is for non-LW gear that will have the tarnation beaten out of it on a regular basis.