If you're going to be racing on dirt, with hard packed surfaces and light dirt on top, then Proline Crimefighters might be the ticket for you. I've been running my Crimefighters (M2 compound, it's a little softer then the XTR they also have) for the last couple months on such a track and I have a good deal of traction. The rubber is still intact too, they show no signs of wear at all.
But if you don't know, then the Knuckles are a good choice indeed. They have more rubber on the ground so they will probably last a bit longer if you run the thing on all surfaces including some pavement.