Both. I feel that in the shows that he's written both music and lyrics, the marriage is PERFECTION. Both elevate each other into a whole different level that no composer/lyricist has ever reached.
His lyrics when he's writing with a composer are great, of course. They're perfect, classic musical theatre lyrics (Except Cool, which I think is brilliant).
But Sweeney, Into the Woods, Sunday, Night Music, and so forth, are beyond brilliant.
Personally, I think I have too much bloom.