You can easily tell also by looking at the engine, since the air cooled has a large can in the back, and the whole engine is surrounded by black sheet metal to deflect cooling air.
The water cooled has not sheet metal, and you can see all around the engine, right to the ground.

But the air cooled it superior.
The water cooled always Porsche Piwis Tester II had head gasket problems, so even when it was increased to 2.2 liter, it still had less hp then the air cooled. The air cooled is a 2.0 l version that is the same are the Porsche 914 engine.
The air cooled is lighter, simpler, had more power, can't have leaking head gaskets, and you can get a total rebuilt engine for only $1299.
So it is a better engine.

The only draw back is worse heat. But you can add a gas or propane Porsche 2 heater, insulate the air ducts, or wrap the exhaust with water filled copper tubing to bring heat to a surplus heater core you add.