I recently signed up and didn’t know where to park myself. So I got accounts on both lemmy.world and beehaw.org.
From lemmy.world I’ve added to a few of the beehaw subs that I’m subscribed to there. But I can tell you I see different content in my lemmy.world feed.
Don’t know why. A delay in the content making it over from beehaw to lemmy? But my home feed certainly looks different in both (even subscribing as closely is I can to the same feeds).
So for now I’ll stick with both and see how the dust settles.
To kind of answer one of my own questions after rereading all the comments here, defederation seems to be per-instance, not from everywhere.
So picking a smaller, well run, non-controversial instance for you account can be an advantage, as you should be connected to everywhere.
The risk seems to be that smaller instances have a higher risk of shutting down unexpectedly and/or not being as well maintained (updates, security etc).