I'm in the same boat as glass doctor in that I've never run the 8xl or 9xl, but I've had a bit of experience in 1/10 scale and with the 7XL. I guess you could say the my recommendation is the result of an extrapolation, but I think it's valid. :)

If I were building a truck or truggy, it definitely wouldn't be with anything hotter than an 8XL. What you really have to consider is the rolling mass of the tires are wheels (and diameter). It's a lot for the motor to contend with, and a higher turn motor is better suited to this. In fact, I'd probably start with a 9XL. That on 4S lipo with the MM esc should make for some worry free fun with good runtimes.