I've used GTKRadiant for years and only recently switched over to NetRadiant (1.5.0 r395) for a Mercy build. As far as I can tell they're the same thing except NetRadiant has some additional bug fixes and features, and the GUI is slightly different.
I have one gripe with NetRadiant though. When expanding selection to entities (eg. a func_group), I can't de-select individual brushes and then delete the remaining selected brushes (which I can do in GTKRadiant). There are work-arounds but the situation constantly pops up.