One thing I realized is that the softer the seat is, the more painful it will eventually become when you ride on it for a long time. I have learned to select those seats which are firmer, because it means it provides you with more support and therefore, less pain after a while. I have actually experimented on a few shapes and sizes of the seat and it's not really dependent on that. It's really with how firm the seat is. Also, I prefer those which are made of real leather because it tends to cause less friction on the skin.