The air in the city is certainly better than it used to be and there are new green areas being opened every year (i.e. the old railway converted into a green corridor, my favourite). But yes like @KHHville wrote the suburbs north (Nanzih, Renwu) and south (Siaogang) are still an industrial nightmare and pollution gets quite bad in winter when the wind brings in additional shit from our friendly neighbour across the Strait.
I agree with @KHHville that the weather in Kaohsiung wins over Taipei big time. It doesn’t rain much and it’s usually sun and (relatively) blue skies after that. Winters are mild, although they can get uncomfortable without a heater too at times, and summers windy without the heat waves seen in Taipei. I guess it has to do with KH facing the sea and Taipei being trapped between mountains. Without counting the building density and the number of air conditioners spitting out hot air.
I plan to visit Kaohsiung, possibly later this year, but for my first year, I think it’s safe to say I’ll probably stay somewhere in Danshui, New Tailand City.
2 major problems are that the concrete soaks up the heat so that even at night the city doesn’t really cool down. Add to that the airconditioners large and small pumping out hot air 24/7. Heatwise Taipei proper is probably one of the worst places to be. Get much breeze in there?