Differences in failure rate between small engines are likely to be insignificant. Kohler has quality standards to which they manufacture. Their reputation relies on having consistent processes and controls to ensure consumer satisfaction. With that being said, 3 additional horsepower and any gain in torque with the v-twin will likely make cutting tall or slightly damp grass (which should be avoided if possible) easier or provide a better cut. Just my thoughts.
As far as reliability goes there probably about equal. A lot for me would depend on what mower I'm going to buy and what I'm going to using the mower for. IF your buying a CC lawn tractor then the 18hp would be probably be fine for it's use. If your getting a ztr I'd opt for the 22hp V twin.
Nothing to avoid about Kohler engines. I ran the 20 single 20 and 22 hp V twin Kohler Courage engines for 4 years and they were a great engine. Never had any issues.
I purchased a Cub Cadet today from a local dealer and I went for the upgraded model that has a few better features as well as a 22 hp Kohler 7000 series engine.
