What are the most important factors in buying a mountain bike?

I'm a long-time committed roadie, but started mountain biking because I was in a part of Mexico with few good roads and lots of mountains. Having returned home, I want to buy a mountain bike for cross-training, but am getting conflicting advice. Hardtail seems ok for what I do. But 26 or 29er? Some say that the 29er is the future, while others tell me to buy a 26 and put the money saved into better components. I want disc breaks, but front and back or just front? Hydraulic or mechanical? My price point is $550-750. Any advice or specific suggestions would be greatly appreciated. (And yes, I know, stay away from the big box stores.)