i feel like ao3 is so glorified. i guess it's easier to find good stories, but the interface and the outdated search functions are so unwelcoming and unintuitive to use.
arguably, wattpad is better as a whole. for the readers, it's easy to find stories you want. you don't have to go through the pain of trying to input search limitations that you don't even know the formatting of.
for writers, the interface is amazing—theres a simple section for the cover, title tags and description, and the formattings for writing the story are easy to use. compare it to ao3's interface that looks suffocating with warnings, tags, additional tags, titles, characters, fandoms etc. it's even worse on cramped mobile screens.
and the freaking search function. on ao3, the slightest typo will reduce your results to nothing. the searches always have to be exact. on wattpad, they dig to find you all the works they can find, and let you easily add any related tags to limit the search.
so, content wise, i'll give it to ao3; most of my favourite works come from it. however, consider this—the reason that wattpad has so many bad fanfics is because so many people, good or bad, love using it.