Dual Blade's Tornado Spin

FA + BS is mostly useful in a non-flat or cramped space, such as the top lane of galloperas or the 2nd and 3rd lanes of skelegons, while FA + TS is mostly useful in a flat, long area where it's beneficial to rush everything together quickly, such as oblivion, bottom lane of gallos, and the bottom lane of skelegons. FA + BS is also more relevant when you need tons of hits to kill something (FA + BS back and forth) or when you know you can 2-hit something like when you're on FC, while FA + TS is more relevant on mid-hp monsters so that you can just keep rushing things around and have them die along the way, meanwhile adding more monsters to your mob. Both skills have their uses, bloody storm might be more useful to you now, but tornado spin serves a different purpose. I would say tornado spin is pretty essential for the long run at least.