I'm a little disappointed there was no option for BOTH. They aren't incompatible and were mostly the same setting during 3e's run (to my knowledge).
That being said, Greyhawk. With that I can still use the planar stuff I want but I prefer it much more as a setting. I prefer my fantasy settings with bits and pieces of the planes mixed in for colourful effect and flavour. I don't need a full planescape setting because I've tried doing a plane game before and it was poorly received and I probably won't do it again.
And as far as Greyhawk, I think the merits of the setting have been raised already in the previous rounds of this tournament and in the recent threads about 5e setting support. I think it is a good, rich, setting and has many little bits to play around with. It also has uninvolved super NPCs so that the players aren't being overshadowed - all things I enjoy, so it gets my vote.
EDIT: Wow. 9-2. Just wow. Early lead, I wonder what it'll look like tomorrow morning.. this morning.. later..?