I have both. Butterfly for two years in an ASH25 and an S80 this summer in a Libelle. Both are better than any equipment I have had previously.

The instant winds on the Butterfly are cool but of little practical value (an exception might be ridge flying). In my opinion, the Butterfly audio has a more pleasant tone. The inertial sensor unit of the Butterfly is a pain in the butt to locate free of significant magnetic fields in the glider (it took me 6 Mo. to get it right), The visual Flarm warning only occurs on the Flarm page and you would never be flying on the Flarm page. The Butterfly inertial sensor unit has to be straight and level with the fuselage in order for the horizon to work properly. The S80 horizon is adjustable in the vertical axis. The S80 has a thermal helper that works fairly well. The tasking software is better in the S80. The cruise audio in the S80 is just weird and annoying. Both require a lot of setup for your tastes and polar, which, of course gives them their versatility. When you update either, much of your hard work setting them up is either corrupted or erased. Inexcusable I say. They both work best as a back up and not as a primary navigation system. And the winner is. A toss up. If they fixed the cruise audio in the S80 I would give it the edge.
DLB