I'll always take 16 in the weapon and 15 in the spare mag over 8 and 7, respectively.
With this train of thought, we'd still be using 5 round, 30-06 M1 Garands instead of 30 round, 5.56/.223 M4 carbines.
Time to move forward and evolve folks.
Was 9 and 8, rounds, but... Are we going to war, or to the Quickie Mart? Do you carry your AR to the gas station, with a couple of extra mags?
Carry what you feel comfortable with.
Why would you carry an inferior weapon when you can carry a carbine, with 30 rounds per mag?