For target shooting at the range, the contacts will most likely be better, while if you're in the field, glasses are preferable. The glasses are easier to find if you drop them, they are easy to wipe "clean", and best of all, you run less risk of getting a lot of dirt into direct contact with eyes.
I tried wearing contacts during one excercise, and it became uncomfortable after a while, especially since you can't really take them out while you take a short nap. Fortunately, I had taken my glasses with me.