I've seen a mate lose his prescription mask overboard, which pretty much screwed the rest of the trip for him. Moreover, I'm in my mid-thirties, where my prescription (I am very short-sighted) is probably going to start 'reversing' to a degree...my optometrist cautioned me that I, like many people, will become more long-sighted as I age. It's bad enough changing my prescription for contacts and eyeglasses, let along the added expense of a prescription mask. Each to their own, of coruse, but I have other scuba toys I'd rather spend the money on!
Contacts are no worries, even if you remove your mask and open your eyes underwater. The trick is close your eyes just before you replace your mask and while you're purging it. Then you can open your eyes, and nothing will have moved. That said, I always travel with spare pairs of contacts - one set stays in my dive bag, and the others in my hotel room/at home/whatever. I've never lost a contact while diving, but redundancy is a beautiful thing.
Happy diving.