Wouldnt it really depends highly on the amount of light on that particular time?
I know that the goal is to set the shutter at double the amount of fps.
So spark is taking 30fps, so you should aim to have around 1/60 shutter speed. Prolly 1/50 will be ok too. This is to get that cinematic effect, too high shutter speed, image will be jittery.
(I havent looked into the spark setting yet, can you change shutter speed to 1/60?? can you change any video setting at all? haha)
But by having that shutter speed, in bright day light, it will be very over exposed (too bright), thus you need ND filter to "darken" it.
At least thats how the theory goes.
So I would say that it is not so recommended to get 1 ND filter since your situation might need something different.
But if you really have to and will only take 1, then probably the one in the middle range? ND16?