Not a fan of discord, it’s good to get a quick answer to a question but I try to spend as little time as possible on there. Once they get any significant numbers of users it’s just a stream of people talking over each other.
Not heard of Mastadon so can’t really comment, will have a look and see what it’s all about
Mastodon is a decentralised social network made up of independent servers organized around specific themes, topics, or interests.
Basically it is cross between a forum on topics like this and twitter. So just dynamic.