Personally, I would work to get both (and did). The Yakooza is a swashbuckler-specific companion, so having him is something that is unique to your class, something special. He also has a fun epic animation. Shiruku has a splendid story quest, and her moves are unique and interesting.
At level 65, I have so many good companions that the Yakooza doesn't come out as often as he used to, but who knows what the future may bring? Shiruku remains one of my top-ranked companions, and I often choose her for boss fights. She's really good.
This is the first time I disagreed with you, Anne. They both stink. The cat ninja is supposed to get a Promotion in the next update, so maybe she will be better then but, Gavin I advise you to wait until you max out your swashbuckler before you pick up ANY side companion. That way they will be higher level when you get them, saving tomes and making them higher level than you can train them until the level cap increase that will come with the new content.