in the last year, I've shot a Bushie, DPMS, Colt, RRA and two S&W. in the military, I shot the M16, M16A1 and M16A2. of the civilian rifles, all of them were fine, but the RRA and the used Colt were the best bang for the buck, literally. the RRA carbine ran circles around the DPMS and S&W carbines; the ban-era Colt and Bushie A3 (modified) were accurate, but not as accurate as the S&W heavy barrel, which cost more than the Colt and Bushie put together.
the most versatile configuration, imo, is an A3 with a 20" barrel (heavy or not is your preference).
you can always buy a 16" upper if you want, and/or a collapsable stock. collapsables are good if your wife wants to shoot it, or you wear body armor, but I don't think they are that necessary for a shooter. same goes for tac rails. buy them later.