I have the 2005 4.4 it gets around 20 on the highway at 70 to 75, it would get better mpg if I slowed down.

The 8 has a six speed trans that also helps it get better highway mpg.

The X5 is heavy and if you do a lot of city driving, mpg drops quick and is less than the 16 EPA number.

The 8 gets better highway mpg and is no worse than the 6 around the city.
To me its a simple choise more power with the same or better mpg, get the 8!