The main distinction is how you access your music.
With iTunes Match, you can download your music on other devices you are signed in to, and get a full DRM-free version.
With Apple Music, downloading your songs on your other devices will simply download an Apple Music version, which is basically a DRM'ed version. Cancel your subscription and the songs are no longer playable.
In either case, your music on your source computer should remain untouched.
Thanks, that's very helpful. A few other questions:
What happens if you cancel your iTunes Match subscription? Will existing matched/uploaded songs still be available DRM-free on other devices? Or will all downloads now be Apple Music DRM files?
What happens if you buy a song while you're subscribed to Apple Music and iTunes Match? Will the song be available on other devices 'permanently' (DRM free) or only as long as you're an AM subscriber?
I'm thinking at this point the safest thing to do is maintain my iTunes Match subscription so that the music I own is always available to me, while using AM just to try out new music (and if I like it I will buy it and add it to my Match library).