I've ran the XZ and the GRP this past summer. IMO, the XZ had more bottom end BUT the best I could get out the XZ fuel mileage-wise from big track was 8ish minutes. The GRP went close to 9....
The XZ was too powerful. Too much power/energy spent on spinning tires. The GRP had more "usable" power for our track.....
Also, the type or types of tracks would also play into the choice. If I had a lot of short straights with massive jumps after them. I'd go with the more torquey XZ. If its not then the GRP...
I will have to say my buddy runs one of my modified V-specs in his truggy he ran it against a few other XZ. The only advantage the XZ had was in the area where you came out of a corner and there was an immediate jump. Other than that, on the straights and longer runs up to jumps, the XZ had nothing on the V-Spec. He wound up easily winning that race....
All the while getting about 9 and half minutes of run time....