Cobia's gonna be kinda hefty for XC racing, but for just XC riding it'd be okay.

I really thought I was going to need a 29'er, but from what I've read...they're not really going to be noticeably faster than a 26'er unless you're riding two hardtails across rough stuff. The 29'er is better on rough ground, but for what you're describing...I'd still go with a 26'er. It will accelerate quicker and just feel quicker. The 29'er would be okay and kinda fun to have something different, but when it's all said and done...I'd stick with 26".