I did a little comparison: reach and reaction on Twitter and Mastodon.

I picked a subject typical of my feed and reworded it to be more native to Mastodon. Same link, issue, people, tone.

With 309 K followers on Twitter it got 81 shares and 179 likes

With 8.5 K followers on Mastodon: 123 shares and 195 likes.

Here are the two posts:

https://twitter.com/jayrosen_nyu/status/1591147769229557761 [305K followers, 81 shares, 179 likes]

https://mastodon.social/@jayrosen_nyu/109326807884220104 [8.5K followers, 123 RT, 195 likes]

Jay Rosen on Twitter

“"He asked: Do you really think we haven’t done enough coverage of the threats to democracy? "I responded emphatically that yes, I certainly did think that." @froomkin's dialogue with @blakehounshell of the New York Times is not to be missed. https://t.co/D1qnw0MkZv”

Twitter
@jayrosen_nyu Actually, I don't think that the two posts are similar. The Mastodon is less contextualized which make your Mastodon post much more understandable. The comparison is not as straight forward as you seem to suggest. IMO
I found the Mastodon post more understandable and relatable than the tweet. To better assess/ compare reach & reaction, you could, of course, post the exact same content on each site.