There's no functional or material difference between commercial receiver extensions and most of the "milspec" receiver extensions out there. Just the same cheap extrusions in a "milspec" diameter.

For durability, a true milspec receiver extension is a different thing altogether - though most of us aren't going to subject our rifles to a milspec-requiring level of use and abuse.

The "milspec" diameter does offer more options for choosing stock bodies, and the cheap ones cost no more than the commercial diameter.