The pin head (pictured) is the Nacra 6.0NA class sail. The 6.0NA came about just before the square head "age". In order to be class legal, for racing, on the 6.0NA you must have the sails supplied from Performance Catamarans....i.e. pin head.

Square head sails offer some performance gains over pin head - but most existing catamaran classes have yet to adopt them for a lot of reasons. EXACTLY why the square head offers an advantage is up for some debate (watch what I just started) - but it certainly allows you additional power up high that is easy and somewhat automatically depowered as needed. We even see monohulls beginning to adopt the square head main sail (did you see the last America's Cup?).

From a sailmaker's view point, getting a square head main sail to function properly can be tricky and it not only involves careful shaping of the sail up top, it also involves precise batten tuning nearest the top of the sail.