XFCE is more user-friendly. LXDE is really fast and responsive on older hardware. KDE is extremely buggy. You might get one stable release then the next update will usually introduce annoying breakages. Gnome is a memory hog and IMHO is only suitable for large touch interfaces. Cosmic desktop is experimental and I won't even bother with it if it is not in Debian stable repos.
Vanity distros with "improved" window managers or desktop environments usually have more bugs than upstream stock Debian. Even Debian, which is an excellent server OS, has annoying bugs in the popular desktop environments. Vanity distros tend to more bugs. They also tend to change default configurations and file locations so users have a tougher time troubleshooting.
You can configure XFCE to look any way you want it to. You can also get a zillion themes at https://www.xfce-look.org.
If you want a fast and stable desktop experience try Debian XFCE and install PekWM and lxappearance for theming. In my opinion PekWM it is the best-designed window manager in existence.
PekWM also eliminates the ugly and non-uniform custom title bars we've been seeing with gnome applications, enforcing a truly uniform look for all windows.
1st choice: Debian XFCE version.
2nd choice: Debian PekWM for resource efficiency and uniform look.
3rd choice: Debian LXDE version for old, slow computers.
4th choice: Debian OpenBox for ultimate resource efficiency.