I think both support the use cases of browsing, sharing, and discussing content pretty well. They just have different ways of categorizing and moderating content.
I'm not overly familiar with Mastodon (hence the typo in my comment), but my understanding is that it's a place to browse, share, and discuss content. The UI does appear to be more like Twitter.
And the Slashdot migration before that. I we really compare what happened to Slashdot and then what happened to Digg, the parallels to what is happening to Reddit are obvious. I assume the result will be the same.
If there was ever a time to promote Mastadon, it's now.