A 9.9v is pretty well a perfect voltage for an upgraded AEG, but I'm against having high RoF on stock guns.
6.6v is going to be way too slow, although the chemistry of the cells is different, you won't get any higher performance than you would on LiPo. Both batteries already provide 100% of the amperage required by the motor, so the regulating factor is voltage.
Honestly I'd rather run a 9.9v over a 7.4v or 11.1v, but like I said, I wouldn't run anything faster than a 9.6v nimh on a stock AEG. Personal opinion