Having run the 7XL in both a buggy and a truck and having run the 8XL in both a buggy and a truck, I can say a couple of things.

First, the 7XL is a beast, or as much of a beast as you can get on 4S; but after having gotten over the initial thrill of the 7XL, I found the 8XL to be a bit more practical. A bit less heat, a bit better runtime, not much given up in terms of performance. I like the 8XL better over all. Besides, 5S isn't outside the realm of possibilities for the 8XL either, if you're thinking about the possible future.