I don't think it's an either or thing.
A song with beautiful lyrics that is sung by an artist in a performance that is technically perfect, but without emotion, may not turn out to be beautiful and the song fails.
The reverse can also be true. A performance that may not be technically perfect but is done with emotion can sometimes make even mediocre lyrics sound beautiful and there fore make the song.
Then too. No matter how good the lyrics and how good the vocal performance, the music for the song, poorly played, can ruin an other wise beautiful performance.
There are just too many variables that can make or break a performance to say it's one or two things that make a song.
Take Care
Tomas