I prefer BSP over WC, but I haven't used GTKRadiant enough to compare. I started out using WC. It's easy to make boxy stuff and simple angles. I made most of the QWPB maps with it: the ksplat series, splatterhouse, pb2dm1 (never made it to q1pb, but it was put in q2). All very simple and boxy maps. It took me a while to get used to BSP. I thought it was unnecessarily complicated, but once I got the hang of it, I never went back. Brainstorm and Midnight were some of the first maps I made with BSP, I think.
3D clipping planes are an essential feature WC lacks. You simply cannot construct maps well without them. I've seen the way DT makes complicated maps with WC and it's a nightmare. If I want to change something, I have to delete like 20+ brushes to get rid of an object that could be made with 1 or 2 in BSP.
I would assume GTKRadiant has 3d clipping brushes, too, and with as long as it's been around and updated, it probably has even more features than BSP. To top it off, it's open-sourced, and I'll probably end up having to use it when we switch to a new map format, so I'd suggest learning GTKRadiant.
