As it currently stands, almost all applications that aren't part of the Gnome project have a GTK 2 version. However, this is likely to change in the future as developers start to take advantage of the newer GTK 3 features. We find that there is little to differentiate between the two, and users that like one will probably like the other. Perhaps, as time goes on, the two desktops will diverge to target specific userbases, but this is pure speculation.
We do know, though, that these two young desktops have rapidly become among the most popular interfaces for Linux. They've done this by listening to users and providing them with what they want.
Best for: Older computers
Avoid if: You like GTK 3
Try on: Mint
In a nutshell: Gnome 2 lives!
Best for: Hipsters
Avoid if: You have an older machine
Try on: Mint
In a nutshell: A traditional desktop
The original refuge to Gnome 3
Many people saw Xfce as a natural refuge for Gnome 2 users when Gnome 3 came out. It's GTK-based desktop environment and has a roughly similar layout. However, many of those who initially joined it after leaving Gnome 2 have since moved on, including Linus. This is because the two desktops were designed for different users.
Xfce's graphics and effects are a little less impressive, and there are less controls. Thunar, the file manager, is also simpler than those in the bigger platforms, but perfectly functional for most uses.
All this comes from the days before Gnome 3, when Xfce carved out a niche as a stripped-down Gnome 2-like desktop for low-powered machines. It does have a few features that some of the even lower-powered desktops (such as LXDE) don't, like launcher panels, but these aren't as graphically slick as in the more powerful environments.
In the last couple of years, other desktops have come to fill in the niche of Gnome 2, while Xfce has continued to do what it's always done well: a simple, low-powered Linux desktop. We wouldn't quite feel right about describing it as 'no frills', but it certainly has a very limited number of frills. Whether or not it has enough is a matter of personal taste.
Best for: Not too minimalist minimalism
Avoid if: You like a high level of configurability
Try on: Xubuntu, Debian
In a nutshell: Aims for simple, but not too simple
The feather weight desktop that still packs a punch
There are basically two ways to design a desktop environment. One is to ask yourself 'how much can we provide to the user?' and the other is to ask yourself 'what's the least we can provide to the user?' LXDE is designed with the latter in mind.
The window manager is simple, as is the file manager. In short, there's no cruft. Nothing that you don't need. The result is a clean interface that's pleasantly free of interference. It's a cool glass of water after drinking flavoured pop. Some might call it boring, and they'd have a point, but does that really matter? Should your desktop environment be exciting, or should it stay out of your way and let you get on with what you're doing with the minimum of fuss?
If you're of the latter opinion then LXDE could be for you. It's enjoying a certain renaissance at the moment due to it being the most popular desktop environment on the Raspberry Pi. The two projects fit together well because they're both based on the principle that computing is about function, not form.