I'm guessing you're talking about the magian katana's, so if you are...
1. What do you use ninja for?
2. Do you need accuracy?
3. Are you willing/able to give up another slot to make up the difference of 15 acc for 19 attk?
I could keep goin on asking questions. But without knowing your gear, or what you use nin for... I can't give you an answer just an estimation:
Since you are asking this question of which is better, I'm gonna take an guess that you're not capped on accuracy. So the dex katana would be superior.
90: WAR MNK RDM NIN BRD SAM DRK PLD BLM
Maat's Cap brought to you by actually leveling my jobs.