i learned about catt in this thread and i never used it, so i may be wrong about something, but these should be the main differences:

- they use a different protocol for device discovery, catt uses mDNS, ytcast uses DIAL.

- catt requires a chromecast to work, ytcast should work with any DIAL enabled device that supports the YouTube on TV app.

- catt can cast from many sources including local files, ytcast merely "tells" the YouTube on TV app to play videos, so it's limited to YouTube.