Focusing on the people and communities, since takes like "Lemmy has no ads" or "reddit has more content" are so obvious that there is zero point in sharing them, I would say Lemmy is better. I think the quality of discourse is a lot higher and people are more likely to type longer, thought out and educated (or well intentioned) comments. If I were to put it really simply, I'd say Lemmy's community is more centred around discussion, whereas reddit is centred around reaction.

However, one issue with Lemmy is that fringe groups and views are overrepresented (particularly left-leaning ones), which can result in an echo-chamber effect in many discussions. I find pile-on attempts, or accusations of fascism, Nazism and right-wing trolling, are a lot more common here towards users who don't immediately join in with a far left circlejerk or attempt to bring a little more nuance or critical thinking to a discussion. Dylan Marron, host of the podcast 'Conversations With People Who Hate Me', once said in an interview that social media pile-ons from people who are actually on your own side hurt a lot more than pile-ons from people who fundamentally disagree with you and I think there's a lot of truth to that. It frustrates me that some Lemmy users shutdown and try to "other" people the moment they have a minor or semantic disagreement with them, instead of taking the time to hash it out or just politely agreeing to disagree. It's kind of ironic that federation allows communities to isolate themselves, yet instead these people remain federated with everyone and then get really offended and outraged when they're confronted with world views that even slightly differ from their own.

But anyway, that type of person is still a minority and Lemmy is, for the most part, a significantly better environment than reddit for polite and intelligent discussion.