Great response Marc!
I have owned both a skyline and a 180sx - and I have found the skyline alot more comfortable and nicer to drive. I had a 1993 180sx, so it had those crappy hessian bag type seats and trim. But if you get post 1994, I think the 180sx comes with velour seats and trim, like the skylines have. The 180sx is also quite simple inside, as you've probably seen, compared to the skyline.... But these are all asthetic qualities!!!
Like Marc said - the 180sx is alot cheaper to maintain. I have found the skyline to be quite expensive after owning a 180sx. S13's have an abundance of parts at wreckers and stuff and they are cheaper on fuel (but this is relative to what mods you do ). But this doens't matter if your not concerned about maintence costs!!