you can buy armor coated optical and even walk on it without any problems. you may find, that once you adjust for output levels, the differences, if any, are rather trivial. Use whatever suits you.
In theory, optical would be cleaner and more pure than coax, but I've yet to hear of anybody being able to notice a difference. Since coax is cheaper, I'd recommend that instead. Of course, if you run out of coax inputs on your pre/pro or receiver, you can always add the optical as well.