The Huna makes you completely invisible, while the Volitak isn't quite as effective (you "fade into the background", meaning you become camouflaged or semi-transparent from what I can gather) but also muffles your sound. For a comparison of the other effect we have the Shelek, which mutes all sound but does nothing to visibility.
The Volitak is your best option of you are sneaking around somewhere people don't expect you to be. Lowering your visibility and muffling sound makes you less likely to be spotted, but if someone knows you're there they'll be able to pick out your blurry shape moving against the background with their naked eye.
The Huna is best if someone is actively looking for you, since you could then stay still and not be found at all. It's also better than the Volitak if you're in a location with a fairly uniform look where you being only semi-transparent would still be very visible, and in places where it's already noisy (such as a factory) so muffling sound isn't important.
The Shelek is best if you are planning to do something really loud without anyone coming to look. Blowing a hole in the wall of a Brotherhood fortress, for example, or silently subduing a Dark Hunter guard without them crying out. (Lewa did this by creating a vaccuum, but for anyone who's not a Toa of Air the Shelek would be a much better option.)
Long story short: The Volitak is is like a mix between the Huna and the Shelek. It lowers your visibility and muffles your sound ,but doesn't make you completely invisible or completely silent.