Xubuntu is just a set of packages for Ubuntu. I use Ubuntu LTS and one of the first things I do on a new install is sudo apt-get install xubuntu-desktop .
Now, if you're on RHEL then Xubuntu would be a bit of a leap, but just installing xfce on Ubuntu is going to be a lot more painful than installing xubuntu-desktop which has a little more functionality out of the box.
What is significantly different? I looked on the xubuntu site and I could not find much information. I did not realize that xubuntu was a lot different than normal ubuntu with xfce4 installed.
RHEL is primarily a server, not a desktop. All the desktops are in the repos already though, and Fedora has offical spins if you don't want to install them yourself for some reason. Gnome, MATE, KDE, XFCE, Cinnamon, it just takes one command to group install.
Now, if you're on RHEL then Xubuntu would be a bit of a leap, but just installing xfce on Ubuntu is going to be a lot more painful than installing xubuntu-desktop which has a little more functionality out of the box.