I am in the process of putting together a LH AR for my boys to use. Bought a stripped lower and BCG from Stag and am going from there. I realize that a RH AR would have worked, but opted for one that fit them more appropriately.
Theres no difference in "fit". Like Tyrone said, a right handed ar has its advantages. A right hander wouldnt know, especially one that hasn't been in some sort of competition with one. No harm in buying your boys one, but if you ever decide to sell it, its not going to be as easy to sell as a right hander. Also when shooting both right and left handed ar's in a session, you have to police your brass on both sides of the bench.