On average they'll be the same. SD first is more likely to roll higher damage (12.5% chance of five hits with one use vs ~1.56% chance of ten hits with two uses) but is also more likely to roll lower damage (37.5% chance of two hits vs ~14.1% chance of four hits).
Using SD first is of course the wiser option if you plan on attacking again afterwards.
Interesting. It seems like Swords Dancing is definitely the way to go in this situation.
-snip-
Thanks!