On the issue M1A1 stocks, the rivets the attach the leather cheekpiece to the metal plate are steel. In the repro stocks they are brass. That's the only obvious difference between them. Also all the repro stocks I've seen are M2 configured. That is, they have the pot bellied front end, the cut for the selector switch and the inner clearance for the connector lever.
While there were apparently some GI M2 cut folders, made at the end of the war, the vast majority were M1 flat forend types.
If I recall, the repros are or were made in Italy. Advantages: Pretty obvious in ease of carry, and storage. Hell, I carried one as a car gun for awhile a long time ago.
Disadvantages: Well, the inner lugs on the folding butt plate will ding up the side of the stock every time you carry it folded. The inner lugs on the hinge wear out really fast, leaving you with a lot of wobble.
My understanding from my old top kicker, who jumped w/the 101st on D Day, was that they weren't meant to be carried folded any time except while jumping. When you hit the ground, you were supposed to extend the stock and leave it open from then on; which is probably why they didn't bother hardening those lugs. Don't know if the repros changed to harder steel or not, but be aware of this "feature" if you go for a GI unit.