definitely 1080 and turn your detail level down to smooth out the edges. make use of the waveform as well to get nice even lighting across your screen.
I had questions about green screen a while back. I shot 1080/25p (you can of course shoot 24p/30p too) and had zero problems keying with either Keylight 1.2 (After Effects) or Primate RT in Motion 4. The key pretty much just popped, so after all the warnings about green screen being hard I was a little surprised.