The difference is different admins, different policy (for example one lemmy could let anyone in, another, like beehaw asks to write why they should let you in, and that you will obey their rules).

There are also things like some settings, for example beehaw disables downvotes, they also don’t federate with lemmy servers that notoriously break their policies.

So best bet would be to choose server which policy fits you the best.

Also some people might want to choose the biggest and most open server. That could be good but because the server is open to everyone it might struggle fighting abuse and also go down because of high load. Such server is lemmy.world right now.

BTW: this might be useful https://github.com/maltfield/awesome-lemmy-instances