Quickleaf
Legend
I think it's fitting that these are the last two settings. They are both excellent settings, and excellent in different ways.
Greyhawk is the most iconic and solid setting of D&D. Planescape is the wildest and most imaginative setting.
Greyhawk is like an excellent steak, while Planescape is an outrageous dessert (probably some sort of flambe).
I voted Planescape, but I won't be unhappy if Greyhawk wins.
I endorse your metaphor They are both excellent (and radically different) settings. I remember the heraldry and adventures from Greyhawk, and DiTerlizzi's art and philosophical intrigue in Planescape. I, like you, am happy to see either win.
As far as what WotC *should* publish for 5e after the Forgotten Realms? I would probably say the original Greyhawk (not the redone version) as setting #2 or #3, which seems to generate the most interest and nostalgia among aging gamers. An earnest update of the Greyhawk setting to 5e without radically changing anything major would be where I'd bet my money.