runtime vs power

[Gundam-Guy]

hey guyz im stuck between geting a 7xl and a 8xl i was wondering what run time i will get with them i know the 7xl will be more power full and the 8 will run longer but is the differance that great between them? i will have them on a mm and 2 2s2p 8000mah lipos from maxamps i would like to do about 40 for 30min i know for run time u will say 8xl but if the 7xl can do like 25min i may go for that please teach me sensei's