For people prone to lots of long outages a liquid cooled 1800 rpm is the way to go.
For those like me with only occasional 12-24 hr outages maybe once a year, air cooled 3600 rpm is fine.
Lots more maintenance on liquid cooled, as there basically an automotive engine. More sensors to cause issues. Harder to find maintenance service.
I service air cooled, but dont mess with liquid cooled. Ill install them, but not qualified to service them. Heck, i dont even work on my own car….i even have dealer change oil.
If you go with a 3600 rpm liquid cooled… better be far from the house. There way way more noisy than any 22 kw air cooled running same rpm.
I installed a 27 kw 1800 unit for a customer about 5 years ago. Man, what a quiet unit compared to my 22 kw. But it has continual, occasional, issues with liquid level sensor in radiator. If it gets even the slightest layer of gunk on it, unit fails. Easy to fix, but if it was mine id find a way to bypass that thing.