One of the things I definitely look for in a manufacturer is history and testing AND verification.
Armalite has good history, has extensively tested there stuff, you can look at their parts and tell they were heat treated
(the weird discoloration/surface "finish" of the reciever near the lockup portion, Rockwell test marks, etc...)
The video Bluegrass armory put out years ago showing them firing their gun with the barrel filled with pipe plugs was impressive!.
My latest Serbu had Rockwell test marks in several places on each critical component.
These are things I personally look for.
I'll beta test a lot of things, but there are some things I want to have an extra sense of security about.
Disclaimer:
I don't know much about Zel, just because I've never looked in that direction,
they may have and do all those things. I do not own any Blugrass Armory products.