In my opinion Gnome 3 is rather slow. I've tried to use the Xfce as my desktop environment but the gtk2 applications were ugly there. If you'll try to change the gtk2 theme it'll lead to the gtk3 theme changing so in this case the gtk3 apps would be ugly too.
I've found the solution for the themes which has both the gtk2 and gtk3 versions (i.e. Atolm):
1. Select the gtk2 theme in Xfce Appearance configuration window.
2. Create a symbolic link like this :
ln -s /usr/share/themes/my-gtk3-theme/gtk-3.0 ~/.config/
For example :
ln -s /usr/share/themes/Atolm-gtk3/gtk-3.0 ~/.config/