@aral From everything I've seen (on Mastodon at least), most users will have a better overall experience on larger servers.
Larger servers are "aware of" more users, more hashtags, more replies in conversation threads, etc., all because more users on the server have introduced them to it to allow it to cache them.
Larger servers are also less at risk of suddenly going offline, suffering from admin tyranny or being arbitrarily defederated by other large servers.