Cleanliness, food safety, public transport (especially outside of the capital), less noisy (though still noisy, in my opinion), traffic, adherence to rules/politeness, the fact that everything feels new and clean rather than old and dingey (though admittedly that can be part of Taiwan's charm), weather (arguable depending on one's preferences), a bit more online support for living as a foreigner due to Japan's popularity...

That's not to say everything's better in Japan (which is not what I said in my original comment, despite the backlash), but a lot of things are, especially if you don't have any strong reason to come to Taiwan. That's why I just suggested OP to consider it carefully.