I've researched this a lot. Initially wanted diesel... Longevity but fuel management required. Then went to natural gas-most common recommendation. Liquid cooled, quieter, runs at lower RPM, lasts longer more expensive... BUT in my environment, the coast it will probably rust out before I get the extra money out of it. So, going with air cooled and in 10 years throw it away and get another.
Variables if you live on ranch and can use the diesel, how much space you have, natural gas access. I decided on Kohler over others.... Multiple recommendations