rschoi, I'm not very familiar with the 3100XL, but I would assume that the 3100 denotes the kv rating of the motor. If so, that is much too high of a kv rating for a truck, especially on 4S lipo. Motors in the kv rating range of about 2000 rpms work the best in trucks.

IMO, the 9XL with the MM on 4S would be a solid, inexpensive setup for a truck. The only real drawback is that the Feigao motors tend to run hotter than better, more efficient motors, like the Lehners or Neus. How hot it gets and how fast it gets hot will depend a lot on the climate in your area and the way you drive the truck.

If you can afford it, you might also want to consider a Lehner motor. Perhaps a 1940/9, hi amp.