When I teach, and train on non-kumite days, I'll use my glasses.
When I get involved with free sparring, or if I know I'm going to be in for a drenching workout, I'll use my soft contact lenses. If I get hit in the eye, the lens could tear, but it's a simple matter of plucking the lens from the eye.
I used to use a pair of hard plastic goggles and polycarbonate lenses. Very tough, and could withstand quite the impact. Sounds great, but unfortunately, if you got hit in the head, that would focus a lot of power on a small surface area. Even though the nosepad was made of softer silicone rubber, it was still a concentrated hit, and it would really, really hurt.
If you can fight without corrective lenses of any kind, then I would suggest doing that. Otherwise, use soft contact lenses. If you speak with your eye doctor, there are all sorts of 1 day lenses that you could use just for this purpose. Something like Focus Dailies, or Accuvue dailies. Just use them for a day, and throw them away.