Been playing around with UDK a bit...
It's definitely a much more powerful mapmaker than GTK could ever be.
Controls are intuitive, once you get used to the steep learning wall (lol). The terrain function could prove to be interesting in combination with Trem, too.
The introduction of Unreal's 'materials' in the place of surface textures is a huge graphical improvement as well. Some 2D surfaces honestly are indistinguishable from 3D realtime renders.
It's also a bit more hassle-free for beginners, I'd imagine too, since you don't need to worry about the void and all in .udk. With the ability to play the map inside the editor, there's no need for 250 map compiles floating around during the designing process.
In fact it's so nice to use that I wouldn't be surprised if a whole lot more people start making very popular maps for T2, as opposed to a select few, like GMOTW, etc., etc in T1.
All in all:
GTK Radiant can go die in a hole now.