The thing is that the average Pink Floyd song is simply better than the average Led Zeppelin song... But Led Zeppelin has more great songs in total.
That's why it's so difficult to pick between the two.
What one could do is look at it this way.
Robert Plant > Roger Waters/David Gilmour (Vocals)
John Bonham > Nick Mason (Percussion)
Jimmy Page = David Gilmour (Guitar)
John Paul Jones < Roger Waters (Bass)
So who is to decide?