I wear contacts too and I know what you are talking about. Riding in hard rain or mud is just a no go. Going downhill fast (specially up north) is just impossible without glasses. Whatever glasses you get try to get the ones that change tint intensity as you ride on bright light or in low light condition. Multiple lenses are okay but you don't have time to change the lense in mid-ride. Instead get ones with adaptalite (specilized) or photochromic lense. Ventilation is a MUST! Also look at the time the lenses take to get brighter or darker.
I had the Tifosi Dalomites and are great for road. But for MTB, the lense intensity did not change fast enough on the trails when going from open trail to fully tree covered shade. I settled for the Specialized Half Time but now looking at the Oakley Radars as I need to get with the times.