Flox's website was only opened up a few weeks ago, so it hasn't really factored into the development of MyNixOS. I haven't had the chance to test it yet, but I'd guess a few differences would be that Flox is targeting enterprise use with proprietary tooling + formats, with a larger focus on the command line interface, in order to guarantee the user experience to a larger degree. MyNixOS targets pure Nix flakes and the Nix command line tool, and focuses more on the website UX to lower the barrier for creating and sharing Nix flakes for beginners.
As the potential for the core Nix technology is so great, many different approaches and tools will be necessary to help discover and cover the different use-cases. Flox made a good impression at NixCon in Paris, and they seem to be doing a lot of good things to make the community solid.