Depends on the music/style you're going for, what would work for it, and the actual quality difference between the two. At least the minimal quality is required before other factors can be weighed, IMO.
I've been in a hard rock/metal band with a technically very competent singer, but she wrote nothing, and sang without emotion/soul (but it was fine for that band). In another band, the song/lyric writer was also the singer, and he was not very good (at least technically speaking), but had a certain power and emotion to his singing that worked for the band. Overall, I'd go with the latter, always...
I've been in a hard rock/metal band with a technically very competent singer, but she wrote nothing, and sang without emotion/soul (but it was fine for that band). In another band, the song/lyric writer was also the singer, and he was not very good (at least technically speaking), but had a certain power and emotion to his singing that worked for the band. Overall, I'd go with the latter, always...