Well, a 7XL will be faster than an 8XL and wont have too much of a runtime diference. If you are going to be running 12 cells sometimes, then sometimes 14, 16 cells, I'd say go with the 7XL, as the 8XL might dissapoint you a bit on 12. However, if you are set on running 16 the whole time, I'd say go with the 8XL. As for the 9XL, I'd only recommend it on 16 cells if you were looking for extra long runtime, but it won't be too much slower than the 8XL on 16 cells.