Hi
The question of whether and/or what to buy is a matter of personal preference.
The Ion-X glass display of the Apple Watch Sport is less scratch resistant than the Sapphire crystal on Apple Watch (stainless steel models), so may benefit from a screen protector. Some thin-film type protectors (such as the Zagg Invisible Shield HD, for example) cover the full display, including around the curved edges.
http://www.zagg.com/us/en_us/invisibleshield/apple-watch-42mm-screen-protector/h igh-definition-apple-watch-42mm
Commonly reported problems with cases / bumpers / guards (in general) include that they may fall off relatively easily, be easily scratched or collect dirt. They may also trap moisture and/or small items of debris, which may damage the watch case and/or display.
Using a bumper / case may also make it harder to keep your watch properly clean and dry. This is recommended, for example, after exposure to sweat and other liquids in order to maximise comfort and to prevent long-term damage to the watch. Some cases may also hinder proper connection with the charging cable.
For opinions about specific products, provided by users of those products, you may find it most helpful to read reviews on the manufacturer's website and/or on websites such as Amazon.
More information:
Cleaning and wearing Apple Watch - Apple Support
https://help.apple.com/watch/#/apd707b42a5e