the problem with a stiff hard soled shoe for MTB is that anything you have to hike with at the minimum, suck, and at the maximum, be treacherous. They have no traction for walking so they slip on rocks. I use a skate-style mtb specific shoe like the Specialized Taho - you can actually walk with it on trails, and hike across slickrock. Plus they have the bonus of the sole being flat and sticky enough so that you can unclip and stand on your pedals (if you have a clip/flat combo like I do) when you just don't want to be clipped in. Make sure you get one that has a full last in it, no flex there.
Imnsho - hard sole mtb shoes are great if you are racing but suck for everything else, unless you ride trails that you never have to get off and walk on.