I wouldn’t worry too much regarding which one you use because they interop with each other. Also worth noting that with Mastodon there’s built in support for migrating your account to a different server. Might be the case with Misskey as well, but not as familiar with it.

And I wouldn’t worry too much which instance you sign up for. One of the bigger ones is better because it’s likely more proactively maintained, but you’ll be able to see content from other servers as well, so you’re not limited to the server you’re on.