I really like my Smiths for mountain biking. Came with three sets of lenses---brown, rose, and yellow. Most of my riding is done in the trees, so I wear my yellow lenses the most because they don't make things too dark and boost the contrast adn visibility of everything.
I've got a pair of Rudy Projects also, and they're definitely better quality optics and fit than the Smiths. But they cost more and aren't as sturdy, so I only use them for road biking. Most Rudys, by the way, only come with one set of lenses. So you have to pay $30 or so to get other tints.
If you want multiple lenses, make sure they're included. If you only get one tint, think about where you ride. If you're always in the trees and don't see too much direct sunlight, I'd recommend yellow or amber. If you're usually exposed, go with a red tint (dark enough to block the sun, yet light and contrasting enough for overcast days).