I go with a spinnier gear on my 29er; 33X20. It's a bit slow on the roads, but on singletrack it's perfect. Plus, you can clear pretty much anything in your path except super steep climbs. I've experimented with various gears, but 33X20 seems like a perfect blend of leverage and speed. Plus, you can "spin like a banshee" so you'll be able to kick your speed up pretty high.