District 37 for sure. If you're not sure of your skill in a desert race, do what I did and start in beginner. That way you get a feel for how the course is laid out, and what they expect of you, without too much pressure from competition. Be sure to pick a good bomb line
I recommend not being stupid like me and starting with the Vikings National. That course is a bit challenging, even the first loop. I got some stories from that one... Sheesh. Loads of fun though, and priceless experience.