2.5d's need 5s. I prefer 2.5d's for truggy action. Thus, 5s.
Aso, the spur takes a beating. Metal is better plastic at taking a beating. Therefore, I use a metal spur without hesitation, like the 46T CR spur. I personally haven't run the numbers on the X2 gear reduction to see how much different it'd be from the X1, but assuming a 46T spur with a 2.5d @ 5s, I would suggest a 13T pinion for starters (and have a 12T handy).