The older ones are better built than the new ones and usually have lovely, original parquet floors and higher ceilings
This is generally true; they also have more character in general. If however you want a second bathroom or a larger balcony/terrace, those are more easily found in new or new-ish buildings.
Your neighbours can be a bit of a mixed bag
Also true. Those very nice buildings (some started in the late 30s, completed during the war) at the bottom end of Żoliborz have excellent flats however many of the residents are older people were placed there by the city, a step away from homelessness and often with alcohol issues. They may well be pleasant enough people, however it keeps prices down in the immediate vicinity. Great when you buy, not so good when you come to resell.
I'd avoid newer buildings with 'leisure facilities' due to differences in taxation rules.