And the aforementioned AR-10 will have an 18 to 20 inch barrel, which works better for 7.62. It suffers in a 16 inch barrel more than 5.56 does.
My own ballistic testing shows that there is minimal velocity loss between a 24" vs. 20" barrel for 308 Win (40fps per inch). I've not chrono'd a 16" for this cartridge, so I do not how how long the drop will remain roughly linear.
But I can tell you that the terminal effectiveness of the 223 suffers considerably when going from a 20" to 16" barrel. The projectile requires velocities >2500fps to produce effective terminal performance. In a 20" barrel this means that the 223 has ideal performance out to 125 yards. In a 16" barrel, this effective distance is reduced to <75 yards.
Between the two I'd take a 308 out of 16" under any circumstances. The new scout rifles by various manufacturers (Ruger, Mossburg to name two) come with 16.5" tubes. This would be a handy and effective package for someone looking for a dual-purpose rifle (hunting, tactical). I already have a SOCOM 16 (Springfield Armory), so I'm set.