If I have multiple niches, is it better to concentrate each topic onto one website, or divide it into multiple, focused websites?
Depends, sometimes different niches are directly or indirectly related and can benefit more being on one website. Remember, people forget you can target internal pages for different keywords and have them show up in search results. You don't need your domain name to only show in search results!
if you divide into many sites, you will spend more time, and energy to take care your sites, it is really hard work.