What I like in UT99:
-Noob friendly editor
-Every actor inherit code from a parent class, makign it easy to change properties and behavior
-Fast rebuild process
What I liked in gtkradiant:
-Ability to merge vertexs
-Stability
-Manipulating brushes was somewhat more friendly than in UED
-Gave up on it because I couldn't stand the extremely long compiling process, specially with light bounces. And I didn't quite understand its hint brushes.