Cases come in three categories:
Cheap, polyester, bargain sub $75 cases. Consider them disposable. They use weak stitching, poor quality parts, etc. I'd buy the cheapest you can find and be glad if it lasts 2 or 3 years. I have one such case that is loosing stitching and looks battered after 1.5 years.
Enhanced cheap cases, in the $75-150 range. These either offer a lot of "features" added to the above cases, such as backpack straps, more reinforcement, more and/or better pouches, more padding, and occasionally, nylon instead of polyester construction of body.
Expensive $150+ cases. These are always nylon, always high quality stitching, always reinforced construction, made very well with the best of parts, best padding. These are lifetime purchase cases. I have a SERT case that is over 10 years old and looks like new.
You want a 36" case for a 16" barreled AR15.