180sx = old, 1980's tech, inefficient, lots of cheap spare parts, cheap aftermarket parts
350z = newer, 2000's tech, more efficient, spare parts are expensive, aftermarket parts are expensive.
both can be as fast as the other, at the end of the day the size of your wallet will dictate what you go with. as for track, if you're getting into proper racing classes, since the 180sx was never sold in oz it may not qualify in a few categories that the 350z will. if you're just getting into it as an amature, use a 180sx and bash it hard, rather than trying to baby a 350z around cos it costs more.