I should have added that gererally a water cooled engine will go more miles than an air cooled engine.... Not slammin anyone's ride it's just that water cooled engines operate under a more consistent temperature thus providing longer life.
That is such a generalized statement, and likely wrong. A properly maintained engine, air cooled or water cooled, has a quite long live. Probably longer than we care to own them for the most part. The killer of most engines is usually operator induced stress, often due to lack of maintenance.
A properly designed and maintained engine, generally has the same service life, by design, whether air or water cooled.
So total engine life is about the same, in general.
However, reliability isn't. Water cooled have exactly the same wearing parts as air cooled, but then add a few extra. Water pump, water pump drive, seals, radiator, thermostat, hoses, sensors, coolant. those extra parts never last as long as the engine. So besides having all the other possible wearing, breaking items as air-cooled, the water cooled has more parts to break. So from a reliability stand point, a water cooled is more likely to break down due to those extra parts.
Most small air craft use air cooled engine because of reliability. the less parts to break, the less likelihood of failure. In addition, a water cooled motor is heavier.
MY Nighthawk 750 requires oil changes and air filter changes, and chain maintenance. Having hydraulically self adjusting valves and no coolant, mean maintenance is simple. there is nothing to fail on it that isn't likely to fail on a water cooled motorcycle. The Engine is known for lasting in excess of 100,000 miles when maintained. The same motorcycle in water cooled, would also need coolant changes every 2 years, hose changes every 3-5 years, and a careful eye on the water pump seal.
If you are talking specific output, then water cooled is far better for making big horsepower. But a mild tune and low HP is also more reliable.
I would buy a water cooled bike, in fact, I like the Honda 919, pretty much a water cooled version of the Nighthawk. However, I have a feeling my Nighthawk, being air-cooled, will actually be around longer than the 919's. As water cooled bikes get older, their cooling systems get less reliable. Hoses harden and leak, the thermostats fail. A water cooled OLD bike is more likely to be overheated and destroyed, in my opinion, then an air cooled bike. That doesn't make the engine itself better or worse, but just that riders are more liely to kill an engine than the engine wear itself out.