Nether one is as popular.
However Mastodon is MUCH easier to use and has better pull as you follow people.. If a person was on Twitter and moves to Mastodon you setup a Mastodon and follow them.
However for Lemmy, if you liked a specific subreddit and not everyone left you search for an equivalent and you find 6 plus (IF your instance is federated with a few even for the search to return results) then you try them out and find little interaction in some etc.
TL;DR following people is easier than creating communities that are active.