I have owned and used tarps from both companies.
You asked about other companies worth looking at. You should add the MLD CF Grace Tarps to your list of ones you are looking at. Pretty sure MLD has been making CF tarps a year or two longer then ZPacks and waaaay longer then HMG.
So here is how this works Michael:
HMG makes extremely overbuilt shelters. Their extra weight comes from larger sized re-enforcements and their custom made guy line that everybody I know removes and replaces.
MLD makes their shelters middle of the road when it comes to construction. They, I and others believe, perfected the method of how to work with cuben fiber, bonding the seams and threading.
ZPacks pushes the boundaries of working with cuben fiber in order to provide the lightest weight possible cf tarps that you can buy. The smallest re-enforcement areas, the least amount of tape and thread, yet still provide a durable shelter for all but the most horrific situations.
In the end, cuben fiber is cuben fiber is cuben fiber. They all buy it from the same company. ZPacks, unlike HMG or MLD offers the ability to have your shelter made in multiple weights of cuben fiber.
I have used 0.74, 0.51 and even a 0.34 tarp from ZPacks. None of them have ever failed me.
I have used a 0.74 tarp from HMG. Without a doubt the most bombproof tarp I have ever owned and never failed me.
I have used a 0.74 tarp from MLD. It was the finest made of all the tarps I have ever used and never failed me.
So really Michael it just all comes down to the sizes of the different shelters, the weight of material that you want to go with, and how abusive you are with your gear.