I would definitely go with the 2.10 size tires. I look at it this way: If you can't go fast and ride in confidence, than your tires are too small. Especially with sand, since a wider tire can float on top. I am a freak when it comes to wide tires, so I would love to have 2.35 tires, but it is a xc race bike, and that probably would be too inefficient (and too big maybe for your rims). So 2.10 it is.
Oh, one thing, I presume you are talking about the Nevegals? Don't go with the kinetics on a xc race bike. They are made more for freeride.
*It kind of doesn't make sense because one would think that a wider tire has less grip, but it is the opposite. The wider tire technically is gripping more surface area of the ground. But in very loose conditions thinner tires dig in too much, and thus cause you to be really inefficient. So the middle size is kind of a compromise, letting you do pretty much everything except maybe not ride in severe mud, and even then they should work. Also the rounder profile of a larger tire will let you corner better. And finally the pros for a wider tire, is the fact that it is like suspension. With a wider tire you can run lower psi and in some cases let the tire float more plus absorb impacts, etc..