Depends on what you really want to spend. NIStune's are around $250 or so, whilst Power FC's around the $800-$1000
NIStune, is just a motherboard, which allows your stock ecu to become completely programmable.
Power FC's are a whole new computer, with hand controller, so you can do 'on-the-spot' programming.
NIStune's are not legal, and neither are Power FC's, advantage of NIStune would be that it is virtually impossible to detect from a coppers point of view, apart from the USB port that is made to allow the USB cable to plug into the laptop to tune,whilst a Power FC is recognisable from its Apex'i casing. Hand Controller is detatchable..but still, if a cop wants to have a look inside your kick panel, you are a sitting duck.
My point of view anyway.