The current ninja combo is either Unji/Unsho or Senji/Unji... and Senji/Fudo for poor people like me who can't afford Unji, but we don't count. >.>
So, offhand choices are these:
Unji: DMG: 32 Delay: 201 STR +2 DEX +4 AGI -1
Unsho: DMG: 31 Delay: 201 STR +3 DEX +3 VIT -1
Perdu: DMG: 32 Delay: 190 ACC+5 ATT+10
So, Perdu has as good of damage, and better delay, than Unji. Unsho is right out, trading 1 DMG for 1 STR is just silly, Jin does not have a STR 75% modifier. Unji loses in the DPS department (9.55 vs 10.10 during latent), leaving only modifiers.
It comes down to STR+2 DEX+4 vs ACC+5 ATT+10. If this was any other slot, would there really be a question? Of course ACC+5 ATT+10 beats STR+2 DEX+4. Unji/Unsho's stats don't help melee damage much, while Perdu's do. They do help WS, but so does Perdu, though probably to a lesser extent. But really, which does more damage, a 700-800 Jin (which isn't garunteed), or the melee hits getting there?
There is a question, though, of if Unji or Senji is the better mainhand with Perdu offhand. And I can't help you there. XD
And yea, what Khajit said... Some S/P user parsed against a U/U, the U/U had better gear, but S/P came closer than it should have, considering gear. So it could be inferred that S/P was superior to U/U.