I haven't used Embassy OS so I can't compare them fairly. I like that they are both open source and anyone can review them. I will say this: the Umbrel license seems like the "freer" one to me. Embassy's license says that if you modify Embassy OS you cannot sell your modified version. Umbrel's license says you can do whatever you want as long as you include a copy of their license in your modified version.

Here are the respective githubs:

Embassy OS github: https://github.com/Start9Labs/embassy-os

Umbrel OS github: https://github.com/umbrel/umbrel-os

And here are their licenses:

Embassy OS license: https://github.com/Start9Labs/embassy-os/blob/master/LICENSE.md

Umbrel OS license: https://github.com/umbrel/umbrel-os/blob/umbrelnew/LICENSE