My take (as a paying subscriber to both, largely because I want to support this world and think both teams are insanely talented and great) is that they're solving similar problems, with some different features.
I wish I could use both with a sync between them until I figure out which one I'd want to stick with. Or stick with both.
Having the same debate - feels like this supports a better variety of sources, but Matter has a better/simpler UI for web articles & newsletters specifically.
This is my question. I've hired Matter as my Read It Later app, and it works great. They do seem to be putting some of their premium features behind a paywall, which is annoying, but I'm relatively happy with the app.