I was actually planning to add this earlier but I thought, even if this was taken into consideration, Twin Arrow is still clearly the winner and the fact that you get Multi-Shot Lv15 and Swift Step: Critical makes it overall the better choice compared to investing more ranks on Ranger.
Giving it a final chance,
Twin Shot technically having 11s average cooldown due to casting animations. (1000%+3200)/11s
= 91%+290 damage per sec.
While Spiral Arrow technically having 38s average cooldown due to casting animations. (1200%+2160)/38s
= 31%+57 damage per sec.

