I don't know if you've made your decision or not. What I've got in my GT is the Fantom .15, and to be honest on dirt it's actually a bit too much unless you're running with all out spiked tires. On the pavement it flies, ya know just point and shoot, but if you're not super carefull on the dirt the dang thing will get all over the place.
So, IMO unless you're a pretty good driver, unlike myself, if you're running on dirt you don't want something with too much power or at times it can be just uncontrollable and frustrating.