i own a 180sx and from my point of view i think it comes down to what you want in a car, not which is better, to make this kind of decision.
an r32 is a 6 cyl, yes, but its still only a 2L, r33's on the other hand are still only a 2.5L. depending on how you drive them, sr20, rb20 and rb25 will all have similar fuel usage.
the main thing that made me decide is the size of each car. you gotta decide if you want a smaller car (r32 or s13) or if you want to go for the bit bigger car (r33).