You can use distrobox to run ubuntu on fedora and fedora on ubuntu.
Imo the difference isn’t too big. If you know what you do, your system will look roughly the same on ubuntu and fedora. Same packages, same workflow etc.
If you keep the base packages constant, i.e. with a immutable distro, you can compare it much better, imo. The experience on Fedora silverblue and opensuse microos will be almost the same for the usual end user. Both are immutable systems, you install packages via flatpack, command line tools via distrobox. System keeps itself up to date. One is standard release, one is rolling.
Flatpak and distrobox offer sandboxing and reproducibility. Imo you want both on a regular install as well which almost make a traditional install like an immutable system, yet you are not as discouraged from installing packages onto the base layer.
If I’d be asked what the difference between fedora and ubuntu is, then I’d say the company behind it from which you get tech support. That’s mostly it.