Dagger and Sword, merged.
What a martial arts practitioner should take into consideration when choosing his next weapon from the bladed-weaponry repertoire, is an armament that can maintain its lethal characteristics (as it is for the dagger, for close-combat situations), but at the same time able to guarantee a good long-range attack capability (which is typical of swords, which have the indisputable advantage of keeping the opponent far enough to make him reluctant to engage combat).
That said, there’s nothing like a sword when it comes to range in blade-edged weapons, although choosing a pair or Sai rather than a sword has some clear advantages:
What you lose in Length, you catch up in Quantity.
Giving up a good long sword for a pair of Sai’s is not always a bad choice: Sai’s are shorter, and compact enough to allow the combatant to carry up to three of them (one per arm, and often a third hidden one). For this reason, a pair of Sai can be easily concealed from view, whilst a Sword might represent a problem if keeping it as inconspicuous as possible is a warrior’s secondary objective.
A good pair of Sai is generally not too heavy for each arm to execute even the quickest movement: we can use this metaphor when comparing Sai to Sword: whether a Sword can be seen as a tall, strong and skilled warrior, two Sai’s can be considered as two short-height combatants: they are short, indeed, but being two means two brains and the unique ability to fight independently, not necessarily join-forced.
The above mentioned metaphors have undoubtedly strengths and weaknesses, as it’s hard to state whether a sword can beat a say or vice versa. The purpose of this comparison is just to show the reader that there’s a reason why, in the past, some Warriors deliberately chose to master the Sai rather than a Sword.
Strategy conditioning Choice.
What emerges from this analysis can be summarized with just one word: adaptability. Choosing a type of weapon rather than another was, at that time, something driven not by personal choices, but factors such as strategy, array of the army, and ultimately the route that laid soldiers to reach the place where the fight had to be take place (hence the choice of a smaller weapon, as a larger – or longer one – could be easily noticed by potential guards).
Sai’s key Features.
What’s unique in the Sai that is hard to be found in other traditional weapons?
Sure the answer is blocking, as the risk taken in getting close to an opponent – even when he is armed with a superior weapon – is easily counter-balanced by the ability to block his attacks – be they generated by a blade or a stick -. If the block leads to the opponent’s weapon to continues its run, the Sai’s unique cross-guard will safely protect the combatant’s hands and, in many cases, lock the blocked blade/stick in a way that only the skilled Sai expert can perform.