Probably all depends on the skill breakdown. Put a crappy value in any of the guard skills that must have single position training (e.g., JR, OD, PA) and it's gonna be a big hit to his value. Also probably depends on what the actual potential coming out of the 4 ball 18 year old is.
If I had this choice, and was set on selling, knowing only this info, I'd probably go 18 year old.