I have found that the XXXS is the best for offroading.
I have run a XXXT, a XXX, and a JRX Pro SE, and though I am a little biased towards Losi's, the B4 is also a strong runner. The decision will likely come down to preference and parts support. You may decide that the XXX is stupid, and that you don't like the color of the wheels, who knows. Ask around, and look for the parts support, and the decision will be easy.