I'm sure everyone's usage is different, but in my experience abrasion has never been the cause of a leak in my air mattresses. It has always been punctures specifically from cactus. I've had my downmat UL7 in some pretty nasty cactus-filled spots and it's never had a puncture. With a sharp cactus spine you're not going to have any better luck with a heavier fabric, heck I've had them go through the sole of my shoe and certainly through the leather on the side. For me the UL was worth it because I wanted lighter weight, but I've never worried about it's durability. Just make sure to sweep your campsite for potential nasties and you should be just fine with any of the UL models.
To answer your other question, if I had it to do again, I would have gone with the Synmat ul9 over my Downmat UL7. honestly that was easy for me, I hadn't seen the Synmat UL9 when I purchased mine. But the way I see it You save some money (though not as much as you think when you consider the schnozzle bag is included with the DM and not with the SM) and you don't have to worry about blowing it up by mouth if your pump device has issues. The trade off is a lightly larger pack size 9×5 vs 9×4. So save $20-50 depending on whether you buy a pump or not, get a pad that's slightly larger when packed but less fussy and probably more insulation if the worst should happen and you have to sleep on a deflated pad.
I'd never had buyers remorse about my DM UL7 until now…thanks (I'm teasing I love my pad, I finally sleep well when in the mountains. It's one of my favorite pieces of gear.)