I think the McMillans are a tougher stock. They don't chip and they have a thicker outshell.
If it's gonna be bagged and not beaten up,, then it's a moot point.
If you want an A3 right now,, check Tactical Stocks And Supply for their inventory list.
I have an A3, and a HS Marksman,, and I like them both very much,, but the McMillan is the tougher of the two, and feels more solid to me. I'll buy McMillan from here on out, unless I get a smokin' arse deal on an HS or I'm building an M24.