ND filter and HSS flash both can achieve same results

That is a bit oversimplified. ND-filters (ND stands for "Neutral Density", which basically means that they [ideally] filter out equal amounts of all visible wavelengths) simply reduce the amount of total EV that your camera gets - with the same settings than before, your picture will become darker.

HSS on the other hand is a flash-/camera-feature that allows for higher shutter speeds while using the flash.

When you need a flash light but also want to stop high-speed action, then an ND-filter will not help you. If, however, you just want to get the exposure right and the flash sync time (usually around 1/200s ± 1/3 EV) is plenty fast for you, then ND-filters of course will work.

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Reflector | fill flash | ND-filter:

• Reflectors are a way to bounce ambient light (e.g. sun, but also flashes or other lights) to a certain region - like a mirror, but less harsh (though there are silver reflectors that are quite mirrory). You use them to lighten up shadows, emphasize regions of interest or add some colors (e.g. by using golden reflectors).

  • Pros: Continuous (as in: no flickering); don't need batteries; cheap
  • Cons: Bulky (especially in windy situations, you will need an assistant); efficiency depends on environment (i.e. don't work too well in new moon nights, if shooting in line with the sun,...)

• Fill flashes are a way to add additional light to the scene. With color filters and flash formers (like soft boxes), they can be used pretty similarly to reflectors.

  • Pros: High level of customisation (many different levels of power output, many different light modifiers,...)
  • Cons: Depend on batteries (bring plenty!); Not continuous, so they need to recharge between shots; need to be synced with the camera's shutter.

• Diffusors are a way to soften down harsh light. They can be useful to reduce shadows, but the diffused area is darker than the surrounding area.

  • Pros: Cheap (typically come with reflectors)
  • Cons: Same as reflectors; Darken the diffused area

• ND-filters, as stated above, simply darken the whole picture (an exception are gradual ND-filters, which only darken down a portion of the frame (they are mostly used to darken the sky in landscape shots). I would not consider ND-filters to be in the same category as the above mentioned light modifiers, because ND-filters at least do not directly affect shadows or other attributes.

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When to use what?

It might be just me, but whenever I work outdoor, I bring all of these with me and choose them according to the situation: There are situations where I use none, there are situations where I choose just one option, and there are situations where I use all of them.

I will search around for a few tutorials on when to use which later (will include them here).

Basically, use whatever you feel most comfortable with at any time - there is no "right" or "wrong" in this (well, using a reflector in pitch darkness isn't the smartest move, but I think you get the point).

Personally, I like to use reflectors over flashes when the situation allows for them - they allow me more flexibility on my parameters. Then again, if I have to work alone, I tend to use off-camera flash(es). I rarely use an ND-filter in portrait photography, but rather use a polarizing filter if there is any use for it (e.g. they are useless in completely overcast skies). On some occasions, I use diffusors, but if you use relatively small ones (diameter below 3m), then they only work in certain situations because - as said - they darken the subject below them quite noticeably.

All of the options work better with an assistant that can help you to reposition your equipment: E.g. it can be really frustrating and exhausting to move between the equipment and your camera multiple times just to get a nose shadow out of the way.