Mastodon

After you join, you can migrate your account to any instance that will have you. You can also have separate accounts on different instances - some people use clients that make it easy to switch between them or even have a combined view of them.

Your choice of server has no effect on your home timeline (only containing the content from people you follow), makes a huge difference to the local timeline (what people on the instance are posting), makes a nuanced difference to your federated timeline (complicated but who your neighbours follow and how your instance is linked to the rest plays a part).

Of course, if you join a less populated instance then finding people to follow is slightly more work.

Each instance also has its own individual stance on moderation and on which Mastodon features are enabled, where optional, or configured, where flexible. Mostly the configuration differences are minor ones around the web interface.