It's not "which is better" that you need to look at, but "Which better fits our needs"? As someone already pointed out for example, storing "Removed by USERNAME" has implications as at some point you may want to have localised versions of that message. You may want to link to that username in the message, and if you later change how users are linked to, you'll need to update all those.
Both your approaches have the two tables needing to be kept in sync, which means you need to do careful transaction work to make sure they don't end up disagreeing. I wouldn't use either, I'd probably have the posts table not know anything about deletions, and the removed table list post_id, deleted_by_id, date etc. SQL will quite happily create the list of all posts with/without deletions for you, you can even build the "Removed by username" message in it.
case deleted_by_id is not null then
'Removed by ' || deluser.name
end as message,
left join removed_tbl on posts_tbl.post_id = removed_tbl.post_id
left join authors as deluser on removed_tbl.deleted_by_id = deluser.au
left join authors user on user.author_id = posts_tbl.author_id
.. as a rough example of how that works.