I only have the same issue with Mastadon that I do with Lemmy, I have no idea what instance to join. This one I'm apart of now for Lemmy I randomly joined when the admin said he'd accept reddit refugees who wanted to try out Lemmy.
If anyone has any suggestions for good instances to pick I'd give Mastadon a try. Otherwise, it's a nightmare for people like me (who's been compared to Chidi from The Good Place in terms of my overthinking and decision making).
Instance is like ISP. It's an important part of the infrastructure, and in principle it does matter, but for 90% of cases it doesn't make much difference.
Unpopular opinion I know. But lots of people who would love Mastodon never sign up because there is such unreasonable emphasis on choosing an instance. Just pick a large stable one, then you can switch if you want later once you get to know the platform.
Regarding lemmy, some instances like lemmygrad, hexbear and in part lemmy.ml are quite against free speech, often removing comments from other instances in threads that are... Politically charged. I would definitely avoid those but can't see much difference between the rest