Battery help



What is the difference of a 9v battery and a battery holder that can hold 6 aa batterys 3 on each side? (it comes out to have 9v) Which one is better for the arduino or any other microcontrollers?? 


The difference between a 9v and 6AA is:

9V batteries offer maybe 1000mAh, where 6 AA can offer 2 times the current of a 9v battery. You will only notice a difference in run time of whatever circuit you have connected to either set of batteries.