I feel like Lemmy has more content, probably because it’s been around longer, so it’s a good base to build from.
I’m also giving an answer to the other thread on Kbin, since I think there are good reasons to use that one as well. They’re both solid, and I’m using both regularly–often viewing posts from one service on the other. For me, it’s “yes AND,” not “no BUT.”
Tusky is best for those who are newer to Mastodon. Visually it’s very clean, straightforward to use, and largely free of bugs (it helps to have been around for so many years).
For users more familiar with Mastodon, I’d go with Fedilab. Despite a few bugs, it’s the most customizable option, with so many features you can tweak to your own tastes.