I would personally opt for a Trance X if I were you. Judging from your weight and description of your riding style it sounds just about perfect. You could go to a Reign, but 6 inches travel for XC is too much IMO, it would just feel sluggish. As for sizing, you would probably feel best on a medium but could ride a small if you prefer smaller bikes. I had a Reign X and it was a fantastic bike, but way too much for what you describe. Hope you find what you're looking for, let me know what you decide.

r u a female?, if so then look at the trasition syren its made for downhilling and made for girls
http://www.transitionbikes.com/Syren.cfm

get a reign x or sx trail. you wont be dissapointed!