I've been going back and forth about this myself. I've used both exclusively in the past and each time been annoyed with one thing or another. Glasses - rain (a light mist is almost worse), getting them dirty, I even broke a pair once on trail. Thank goodness I had a tiny tiny tube of gorilla glue with me. Fixed them right up. Contacts - same problems everyone else have already stated.
Thinking about trying the monthlies this time. I really want to wear sunglasses this go around and not worry about them being an expensive Rx pair.