I've been a fabric nerd since childhood. Ever since I watched the episode of 'Mr Rogers Neighborhood'  where they visit a fabric mill.

Recreational Fabric 101

The standard way of measuring a recreational fabric is by the thickness of the yarns it is composed of. This is known as ‘denier’. 

Larger the denier, the larger the individual fibers. That translates to (all other considerations aside) more tensile strength and more abrasion resistance for a given fabric composed of those individual deniers.. But also more weight per square yard of fabric.

Xpac X21 for example uses 210 Denier fibers. X50 uses 500 Denier fibers, tx07 are 70 Denier fibers etc. etc. (this is just for the face fabric, other considerations shortly)

Next we have to understand that the composition of the fibers and the weave of the fabric can have an effect on fabric performance. 

For example, ‘ripstop' refers to a fabric which has at various intervals a reinforcement of thicker or stronger fiber than the base fabric fibers. This is an attempt to isolate tears in the fabric, while keeping overall weight lower than a full fabric of higher denier. You can spot a ripstop fabric by the telltale 'grid' it contains.

Differences between nylon, polyester, dyneema, and other fibers exist, as well as subtleties in types of nylon (Robic for example is a proprietary nylon formulation that exhibits marginally better tear and abrasion resistance vs regular nylon 6 of the same denier.) Common Dyneema® Gridstop, has a base nylon fabric with 210 denier fibers. The Dyneema(ULTRA-HIGH-MOLECULAR-WEIGHT-POLYETHELYENE!) used in the ripstop is such a high-tenacity fiber that, used solely as a ripstop thread, it has a significant effect on the overall tensile strength of the fabric. Cordura®is a brand of nylon-based woven fabrics, with many different blends of fiber content and differential weave.

Cuben Fiber is unique and has shaken up the game a bit since it’s been around. Cuben is NOT a traditional woven fabric. It is a mylar/dyneema sandwich. The 'hybrid' cuben materials used for packs are only marginally more abrasion resistant then their face fabric (50d and 150d respectively). However, where cuben shines is in its tensile strength. It’s hard as heck to tear the stuff apart.  Though, truth be told... most wear on backpacks comes from abrasion. (rubbing against rocks, trees, boxcars etc).

Next we move onto coatings. Traditionally, outdoor fabrics had a polyurethane(PU) or silicone coating sprayed on them. This made them waterproof(depending on the thickness of the PU/sil coat) and increased their strength. Problem here is that the coating wears off in time, with UV exposure etc, peeling away and becoming useless(and potentially messy).

Xpac and cuben are unique in that they do not have a PU or silicone coating sprayed onto them for waterproofness. instead they are laminated with a PET(mylar) film. This gives them a much higher degree of PERMANENT waterproofing. Also, increased strength over a PU coating.

WOW! that was a lot!!

I’m still leaving out ‘hand’ or the feel/stiffness of a fabric, and some other minor things but I don’t want to rant too00 much...

To synthesize for someone who needs reccomendations.

A 210 denier fabric is great pack fabric for nearly everything you can throw at it. With extra hard use such as climbing GRANITE, or anything that will se a LOT of abrasion. Getting more than 210 denier starts to make sense. 330 denier is great, 500 denier is  more greater still( ;) ), though heavy.

I would consider Tx07 a budget alternative to the 50d Cuben hybrid. Take care of it and it'll last a long time. (or great for rolltop closures or less hard-wear uses).

X21 is my favorite all around pack fabric, hands down. X33 may replace that if I shred my X21 bag climbing this year. Hasn't happened yet, though.

X50/51 are WAY stronger than any cuben materials folks use for packs. Great for pack bottom reinforcement.. Or for something you want to soften your landing when jumping off of a moving freight train onto ballast and not worry about.

Xpac fabrics, specifically the X- series (without the polyester scrim liner)are the best of all worlds as far as I am concerned. Lightweight, waterproof, inexpensive and great strength characteristics per weight. With their laminated PET layer, these fabrics can be used to create seam and reinforcement tapes, and take Cuben seam tape quite well.

Heck, if you can afford it are looking to save weight, and want the tech factor(ULTRAHIGHMOLECULARWEIGHTPOLYETHYLENE!!1) go Cuben. I've put my cuben packs through plenty of abuse, and I'm very happy with how well they've held up given their light weight. Construction methods help quite a bit there too ;)