I have all three, and they're all quite easy to use.
Like he said above, you don't need to buy Q3 to get Q3Radaint, it's availbe at
www.3ddownloads.com)
And yes, they are almost identicial, but Q3Radaint will allow you to turn off Snap To Grid (Which EFRadiant didn't support) and that's pretty much it.
GTKRadiant is almost exactly idectial to Q3, except that the interface look like java.
EFRadaint is bacailly the same but no snap to grid avaible.
I use Q3Radaint personally :)
*fires up map*