AO3 in terms of net quality and the smut is better. The early days of fanfic porn had people using the wrong shit as lube, especially when writing male/male fics. That largely has been fixed, with Lube Being Everywhere, but in it's place that ABO shit has taken over. The Troon shit has increased, too.
The demographics of those who write fanfic are overwhelmingly white women, though there have been more Chinese women getting involved. AO3 is blocked in China, and you need a VPN to use it. In any case, it's women, white "enby" women, living out their fantasies through reading two white men (majority of same sex pairings there feature white men, go figure. Voltron and Shadowhunters, or the Untamed or Kpop bands, are all mixed or Asian). They know what men are.
With the blocking button now implemented, and ability to shut off comments entirely, authors truly remain in an echo chamber. Fanfic politics has become more entrenched, and you'll find pro shippers supporting literal pedophile organizations like Prostasia or telling TERFs to go die in a fire. The discourse is dominated by Jews (go figure) or fat black trannies. I have a fuckton of receipts, but you'd get the most from LizCourseRants, who compiles FBI style hand books on people she doesn't like.
FFnet has a shit interface, but it is great for it's private message system AND permanent reviews. It's the Yelp of fanfiction. AO3? Authors can curate comments, so you'll never see a bad word.
Edit:
here is Lizcourserants speaking positively on Prostasia.
https://archive.md/bIqt6