As far as I know, UIs are like Distros: each person wants something else.
All of them should be able to run your favorite applications, allthough your choice might depend on them.
If your favorite applications are gtk-based, they are likely to integrate well in a gnome environment.
If your favorite applications are qt based, they are likely to intergate well in a kde environment.
If you're memory regarding, the "negative" aspect of mixing gtk and qt based apps is that you use more memory than you would if sticking to a single library toolkit.
Other than that, your favorite apps should work on any of those environments, as long as you installed their dependencies.
I love KDE, my father swears by xfce, my mother can't use something different than unity.
I guess the easiest solution is to install kde, xfce, e17, fluxbox, gnome, ... on your current ubuntu system, test them out (you can choose which you want to use at the login screen). When you'll have tested them all, you'll know which one is yours --> desinstall the others.