For a realistic perspective, I study old western martial arts. The Italians had some skill with making longswords (Im not talking about rapiers, dunno why people confuse the two) And depending on the weight of the longsword, I might keep a Stilletto in my off-hand.
A Stilletto is a long Italian knife with no edge. It is only used for stabbing and blocking and the point should be kept very sharp. Its best used when aiming for the ribs. It easily slides between them and does gratuitous damage to the intestines.
You know, come to think of it, there was a forum discussion on what kind of weapon the wrist blades in assassins creed are. Im wondering if it is a possibility. From what I have seen, Altair/Ezio dont seem to slash or cut with them too much. Usually just lunge and stab....