Hi
The sapphire crystal on Series 2 models is more scratch resistant than Ion-X Glass.
The decision regarding whether or not to use a screen protector or a case / cover is a matter of personal preference.
Some thin-film type screen protectors cover the full display on Apple Watch, including around the curved edges.
Potential problems with cases include that they may fall off, become easily marked and/or collect dirt. They may also trap small items of debris (which may cause scratches) and/or moisture.
Using a case may also make it harder to keep your watch clean and dry, which is recommended by Apple after, for example, exposure to sweat in order to maximise comfort and prevent long-term damage to the watch.
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