Jay Lerner-Z said: "Not a very insightful answer, but I think he was equally good at both. If I had to choose, I'd say music. Some of his lyrics exist only so he can indulge in his love of language. They are not there for any other purpose."
While I don't agree that some of his lyrics were just there for love of language (though, yes, he loved word play, and was an unparalleled practitioner of it), I agree he was about equally adept at both and I agree that his music has a slight edge.
There is for me, at times, a sort of...how do I say this and not offend anyone?...mechanistic quality to the lyrics he wrote for other composers: West Side Story, Gypsy, Do I Hear A Waltz?, and even for his own music with Forum. I know that "mechanistic" isn't the right word. "Impersonal" would be wrong, too, probably. But I don't feel an emotional engagement. They feel like five finger exercises. And I feel like, if he could do what he's doing lyrically in all of his shows better than anyone else, other people could still have done it. His music, on the other hand, is simply unlike anyone else's, and to my mind overflows with emotion.