NOS is a (relatively) inexpensive way of getting power, compared to adding a turbo.
However, it is in no way a substitute.
It depends on what you're looking to achieve - if you want cheap power for a 1/4 mile stormer go for the NOS. If you want to build a properly quick car that won't just be a hit of power every now and then, build a proper low compression engine with a turbo.
Thing is, the latter will cost you about ?4k more!!!!