I have USGI and Korean mags, all seem dependable. I've heard stay away from Pro Mag, but I can't say that it's a viable warning, I haven't tried any.
On the 30 round mags, if maximum reliability is wanted, then simply install the M2 magazine catch, which was designed to address the issue Barkoff mentioned. It has a 3rd contact point that stabilizes the magazine. This is actually most important on the M2 Carbine in full auto, I've never had an issue with 30-rounders in a regular M1 Carbine and original catches.
With all that out of the way, the rifle was initially designed with a 15-round unit, and it's the one I prefer. Like 20 round mags in an AR, it's just easier to carry/handle the rifle and if shooting off rests, much easier to do with a 15 rounder.
I really enjoy shooting the carbine. I have an Underwood made in '43.
