Adam,
both cars are great cars. Depends on what you are looking for.
180SX has the SR or CA engine depending on which year. They respond really well to mods. 180SX are much lighter, if i'm not wrong they weigh about 1240kg? and the R33s weigh 1340kg.
The skylines are 'grand tourers'. Much bigger interior, bigger engine, more torque. On the hwy when the 180 sx is at 120km/h, it will struggle to increase more speed whereas the R33 will keep pulling away even at 140km/h...