I'm thinking about joining mastodon, but I don't know what server to go with, and the list of servers seems to have very little information about each. Right now I think I'll open an account on mastodon.social.
I'd prefer a server with a high character limit and that has access to most other servers.
I would join infosec.exchange of the ones mentioned. Or just join mastodon.social. It's not going anywhere and it's the default and it's likely going to be the most stable in terms of user experience and longevity.
Having said that, to a degree it doesn't matter? Just follow the hashtags that you are interested in as soon as you get there and it should create an instant active feed. And don't forget to tip your server (I recommend kicking in a buck or two a month to the server admins and same to the mastodon devs.)