It's a blurred boundary, but essentially, you're looking at shorter travel (100mm), steeper HA, lighter weights, and obviously as you get lighter, you get more expensive. I don't expect race bikes to be less robust, although at the very high end extra weight saving measures can be taken in consideration of the rider's weight, as a lot of racers are quite light anyway. But you're in custom territory by then anyway.
If you're thinking of racing XC, go and race whatever you have at first and develop your own ideas of what will work for you as a race rig. It's not going to be the bike that holds you back at first.