I'm going to go with Elton. Madman Across The Water and Yellow Brick Road are just damn near perfect albums. And while I love the early stuff Phil did with Genesis (the Gabriel years up through Duke), those songs were much more collaborative efforts with Gabriel, Rutherford, Hackett, etc.. The solo stuff from Phil and later Genesis stuff really doesn't do much for me. Seems to me that Elton collaborated exclusively with Taupin and Bernie pretty much just wrote the lyrics while Elton wrote the music.
Now if you were to compare Elton's skills on piano versus Phil's skills on drums, I'd give Phil the edge due to his prog years in Genesis.