They are both different things really, it really depends on what kind of noise problem you have….. the hum debugger works great for removing guitar pickup hum, the Boss NS-2 is a gate which is better for cutting out pedal hiss. They both have downsides, like Owen said the HDB will colour your sound a little and a gate will be a nightmare when switching from clean to distortion (because it’s hard to set it up to work with both)

.. a better solution imo is to find the noise source and try to fix that, fit quieter pickups, buy new cables, fix the earthing problems your gear has… or whatever it needs.