Racing: If you're talking about actual racing on tracks, they usually have pretty defined racing classes. The stock short course trucks are popular where I live (Slash, SC10).
Off Road/Bashing: I think any of the above mentioned could be fun off road. the Slash and SC10 sit a bit lower, and tend to have more trouble in grass (in my experience), unless you were to adjust the suspension and raise them up. The Slash is pretty durable. There are several Stampede fans here, and from the footage I have seen, it looks pretty fun off road.
The XS is an unknown for me. Again, from the footage I have seen, it looks like a lot of fun. I just ordered one, so I'll know more soon