@ExtraCortex: It's all been said. Just know that BSP is ideal for indoor levels and ABT is better suited for exteriors. A level built using BSP takes the Map Compiler longer to build/create because it has to create portals and calculate lightmaps (and other behind-the-scenes simple tricks and nonsense). An ABT level will only require lightmap calculation; however in my experiences with the private beta it takes 10 times longer (literally) for certain levels to build when using this method than a BSP test build. However, I think the ABT build will be 5000 times faster than a BSP final build.

ABT also allows you to use any kind of geometry you desire, so there are no concave block errors. However, the new pro-only BSP is supposed to eliminate this at some point in A7. That'll be nice.