I don't have any actual experience with this stuff, but it just so happens I've been reading up on this yesterday and today (I'm thinking of making some bags too). From my research:

X-pac is lighter, more water resistant and holds less water than Cordura but is not as abrasion resistant. Xpac colors fade pretty quickly from UV but (from what I've read) isn't structurally degraded. X-pac water resistance comes from the middle layer, so doesn't really rub off. X-pac doesn't have any self-healing properties for pinholes so seams tend to be porous unless taped.

Nothing beats Cordura for abrasion resistance except ballistic nylon.

Caveat: "Cordura" is a trademark name that refers to a range of products made from different materials (nylon, polyester, etc.), so there's some vagueness when talking about "cordura", but I think the nylon version is what most people are talking about when they say "cordura".

I'm designing a half-frame bag and a big Carradice style saddle bag and am leaning towards X-pac VX21: Terrain X-Pac? Laminated Ripstop Fabric

I didn't look into/compare cotton canvas, the traditional stuff used by Carradice, Rivendel, Gilles Berthoud, etc.