The 6 cylinder engines tend to be quite a bit more expensive to
maintain than the 4s. - 50% more things to go wrong, extra plugs, etc.
Both Lycoming and Continental have had more recent problems with their
6 cylinder models than on the 4s. Within 4 cylinder models, depending
on where you live parts availability may be an issue for Franklins and
Rangers. Whether it's a 180 or 200 hp Lycoming matters little, though
you will appreciate the extra 200 hp climbing and you can always
throttle back in cruise to control fuel consumption. So for sheer
practicality, I like the 4 cylinder models if they will provide the
performance you need.

Having said that, a 6 cylinder engine with a 3 blade prop provides more
power and speed with less vibration.