Sapphire crystals are fine. Its transparency and resistence are remarkeable. On the other hand, sapphire glasses use to produce some glare - mainly the flat ones. It's not common to see domed sapphire watch crytals. Of course, they're expensive, harder to replace and I don't think they can be polished - well, if ever needed.
Plastic crystals can always be polished. And it is quite easy and inexpensive. Even deeper scratches can be removed with a proper sandpaper and, after that, polished with ey can specif plastic polishing compound. Good plastic crystals are also nice! The Omega's trademarked hesalite is a good example.