The tail shape depends on what you want the board to do.

The squash tail is great for generating speed.  When you pump the board for speed the corner of the tail digs in and give you a little more pop.  The downside is when you turn the tail digs in again and slows down your turns.  If you are the type of surfer that is constantly pumping to gain speed this tail works well.

A round tail you will lose that extra pop but maintain speed better through turns.

The swallow tail puts a break in the tail block of a squash tail.  This allows water to flow around the tail when turning.  This maintains a little more speed in turns than a squash tail.

As waves get bigger, your board is traveling at greater speeds you need to pull in the tail to maintain control.  Most surfers will transition to a round pin.  This makes you tail narrower and fin cluster closer.  This will give you more control.  Another option for pulling in the tail is wings.