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I'm not a big fan of the glass ones that adhere to the screen because they are difficult to remove if you break them. Since they are so thin, a minor pin point impact will break the glass, and possible damage the camera's built in protector. To remove the broken glass, you often need a hair drier to heat it up for the adhesive to release.
There might be some out there that don't break as easily, or do not require a hair drier to remove, but I really haven't looked into it much. I use the Kenko multicoated screen protector on my D800. It's a thin plastic protector that guards against fine scratches. The Kenko kit contains one for the top and rear screen.