A few things I’ve noted.
8 Batteries seem to be oriented to the use of rechargables. Most are 1.2V vs 1.5. 1.5×6=9, 1.2×6=7.2, so the extra 2 batteries gets you back to 9.6V. So it will feel more like regular alkalines.
AA vs AAA This is about capacity, or run time. The AA batteries can hold more mAH, my recharges AAA are 1100mAH, and AA are 2800mAH so in theory/simply put, the battery pack w AA should last twice as long as AAA.
From what I remember reading, the official LEGO rechargeable should be closer to 9V, so less of an issue.