All AR-15 platforms are mil-spec guns
Not true. Given, all AR-15's will get the job done; not all are full mil-spec.
You will pay about $200 more for a full mil-spec AR-15.
Mil-spec are generally overbuilt. Some custom AR's actually exceed that rating (SS barrels, SS Bolt assembly, Billet uppers/lowers, etc)
Mil-spec is just a standard that some companies use for marketing, where as other companies use customer feedback for the reliability and performance of their weapons.
If you get mil-spec, you'll be happy with it; if you don't, that's fine too. Some AR-15's exceed mil-spec requirements, but they just don'e advertise it. The way you'll be able to tell is the price you're paying for one (usually the case)