I'm making the frame out of hollow aluminum tube and a foam pad, the tube and pad will be removable, the hipbelt as well to save weight depending on what I'm taking inside. I want to save weight on the fabric now rather than be sorry I didn't later.
I could use the coated silnylon from thru-hiker on the sides and 70D at the bottom then sealing all the seams with silnet, or using the heat sealed fabric Terry suggested and going 30D on the sides and 70D at the bottom.
The maximum weight I would ever carry on the pack will be around 40lbs, but most of the time with maximum load of food water and fuel will be around 15-30lbs. The pack only needs to be waterproof against rain, storm, snow etc... no full submersion, I have yet to drop a pack in any body of water.
The X-Pac seems very tough but I don't know if being 5 times as heavy as silnylon for being extra strong is worth it, kindy like taking an extra flashlight or something that is "just in case". I'm no expert in such fabrics though, so I could be way off.