A bit off topic, but the usability is the same, since you can customize both to the same degree. And I have yet to hear a single valid reason why Linuxmint continued to do XFCE.
I have no idea why Mint continues to do XFCE but I can tell you why I use it:
From the blog post: http://blog.xfce.org/
Technically gtk3 is nothing different then gtk2 when it comes to programming. The hard parts are porting of some custom widgets (drawing and size allocation), replacements of some deprecated symbols and link to gtk3 libs. All things a user is not going to notice if we do it right.
Gtk3 is also not faster than gtk2, maybe there are some areas were it got a bit faster, but so there are areas where performance decreased a bit. Nothing shocking here.
An issue I’m aware of is theming issues in gtk3. From what I understand this changed back and forward in gtk 3.0, 3.2 and 3.4. So we need to decide which version we require to get this working consistently, because people will complain if only the Raleigh theme can be used
From the Xfce point of view there is (again) the resource problem for porting all plugins, because if for example the panel is ported to gtk3, also the plugins need to be ported. Not all goodies are maintained, but usually they work and distros can compile them. If in 4.12 suddenly 50% of the external plugins are not working that will be another thing users will notice.
The difference in my mind is:
(1) A peculiar focus on the end user.
(2) They do not appear to have the Gnome-intes' obsession with equating every Gnome2 feature ( really any feature ) as a bug that needs to be eradicated.
(3) They did not spend valuable resources creating a user interface that is a Proof of Concept project to convince parters that it is the perfect UI for a TV: http://www.ubuntu.com/devices/tv
(4) A level of adult supervision that looks upon the whole of XFCE rather that it's constituent parts in isolation that is severely lacking with Gnome.
(5) Doesn't say silly thing like "gksu leafpad" is unsupported because you shouldn't be using a GUI application as root.
Now it appears that there's only 3 people working on XFCE - The blog author, his brother Darryl, and his other brother Darryl - which no doubt forces them to think this way but I think that this has turned out to be a benefit.