Just anecdotally, I think flatpak uptake on the dev side is much more significant than snap uptake (i.e. flatpak seems to be "winning").
I've seen a lot of FOSS software that offers official flatpaks, and many programs are suggesting a flatpak install as the recommended or default method of installation. Flatpaks are also integrated into, for example, the GUI "software store" that ships by default with large distros like Debian and Fedora. I generally prefer distro packages, but I've installed a few flatpaks where there was no package for my distro, because the devs made a flatpak available, and it worked fine.
Snap adoption seems much lower, and I've never seen software that actually recommended installing it via snap (although I'm sure there's some out there somewhere). It seems like an "only on Ubuntu" type solution, which makes sense for the reasons Hendrik mentioned, but is bad news for a supposedly universal packaging format.