Liquid cooling are
required if you want to (re)build an outboard motor with it, because the motor will be enclosed in a confined space - and there's already a waterpump embedded around the propeller-axle.
Liquid cooling could also be preferred on a racing/cross-bike with a radiator/pump to cool the motor, due to the heavy load. (In short; if the motor would be embedded inside a confined space, or run very hard; Liquid cooling would be preferred.)
If you are just going to mount the motor on a bike frame for street-use, an air-cooled motor would suffice.
Edit;
It's the same motor, but one has cooling fins for applications where good airflow are available, and the other one are encapsulated with an enclosure where water can flow to cool the motor.
