I prefer snap caps and bonus points if it snaps to post. Other types are acceptable unless the screw cap takes more than 2 turns. I love the ease of opening some capless but haven't found one I enjoy to write with. Screw to post, especially on a short pen is a deal breaker.
I have not found cap sealing to be correlated to cap mechanism. I don't like any cap that allows my pen to dry up in just a few days.