With RPi OS and Ubuntu both being Debian-based, the main difference from my perspective would be Ubuntu's support for 64-bit userspace.
BTW, I don't use the Pi's GUI, so which window manager it's running is irrelevant for me. Therefore, I don't know if Pi even supports any other options than its default. When I'm using an Ubuntu desktop, I run KDE.
Ubuntu server (32 or 64 bit) is already available for raspberry pi, which is more or less the same as regular ubuntu except it doesn't come with a graphical desktop environment. From there you can (in theory) install any DE you want, assuming you want one in the first place. I had some mixed results though on my Pi 4, when I (very briefly) tried out a few DEs.
I started with KDE, as that's the only DE I have experience with. I think it worked as long as you disable the compositor, but performance isn't great. Gnome was similar IIRC, worked but slow. I also tried Xfce and LXDE/LXQt (can't remember which one, maybe both). Those seemed to work OK, but didn't seem to recognize my TV resolution properly (1080p) and their resolution settings apps were buggy and I couldn't change anything. Maybe could have solved that using CLI with xrandr commands or something but I didn't feel like messing around with that.