Carbon is seen as expensive and light while Aluminum is heavier and cheaper. Both are, to all practical purposes, more than robust enough for the job. At the price point you are looking at (for a hard tail), Carbon is a no brainier and superior in every way.  

If you are worried about failure mode, both are as likely to fail catastrophically as each other, although for different reasons. Aluminum will fatigue and stress crack then fail, while carbon tends to fail from impact damage.  

Do not worry about the frame failing, it happens, but to be honest, I have seen plenty of guys in bandages, casts and hospital with fully ride-able bikes, and have never personally met someone who got seriously injured breaking a frame.