There are a lot of different X Pac fabrics and there are a lot of different type Cuben fabrics.
Perhaps the first thing that helps on comparing pack fabrics is the base weight of the fabric itself. I will generalize the following info so you can see a little of what I mean- of course many pages could be written on this and it would still not be perfectly clear....oh well...
The Dyneema X that we use - and similar dyneema gridstop fabrics used by many others- is about 4oz sq/yd
Some lighter dyneema gridstop is in use at about 3oz sq/yd but it's less than half the strength of the 4oz and really only save a couple oz's in the finished pack.
The Cuben that a few smaller pack makers use is about 3- 4oz sq/yd- but some use heavier styles for some packs - especially burly climbing packs.
The X Pac style used is mostly above 4oz sq yd. except in the MYOG circles.
Overall- each type of pack fabric has it's pluses and minuses- nothing is clearly better in all areas IMO.
A few criterion that go up and down in desirability between them all for an end user includes Durability (tear and abrasion), Water Resistance (either as a coating or other means), Price, Repairability, Color, Sustainability, Cost per day/Service Life. etc.
For a manufacturer also add sewing/construction ability/issues, dependable availability issues and over time the fabrics quality from run to run- I guess those are all mainly price issues for the end user too.
SO, if you take those fabrics in the 3-4oz range as a way to make it more apples to apples- then you can start to see some tradeoffs that make sense. For example , we have made lots of custom packs from X pac but in that weight range we think the Dyneema X is overall better- lot of needle holes in light X pac is problematic for tearing along the lines. The cuben in that wt range is very strong but expensive and I think the abrasion is bit lower until it get to the 4+oz range. Some people think the Dyneema X WP coating wears off too fast- but thats an opinion too- I know many who get multiple thru-hikes out of them just fine.
Personally, I'm thinking that if a UL 16oz $175 pack lasts 2-4000 miles and 100-200 days on the trail ( I know many of our users and other UL pack companies too get that longevity all the time) maybe that's a good deal at a bucka day for a special UL pack. Others may disagree and want 50cent or maybe 10cent per day return on investment.
In general- the heavier the pack the longer it can last assuming the base fabric is heavier and all else on the pack is the same.
For a well made SUL / UL 16oz pack what do you think is a fair cost per day for use over a reasonable service life? $2, $1 , $.50, ?
EX: Would a 6oz overall increase in a 16oz pack weight be worth bringing the avg cost of use per day down $.30?
How about the reverse? Would an increase to $2 per day cost to use be worth a 16oz pack dropping 6oz?
Everyone will have their sweet spot for the equation!