It all depends on the kind of power you are looking for and how long you are looking to run it.
Nitrous is cheaper at first but if you are going to be opening up that bottle a lot, factor in bottle fills. (With my discount it used to cost me $20 every time I wanted a 10lb bottle filled) Also, you do not have the power all the time.
With a supercharger, the power is always there, whether you are at the track or playing with a Mustang (ahem) on the highway.
Both are safe with a good tune.
Personally, I would go turbo (haha) but second choice would be supercharger.