Obviously (to me, at least) neither instrument is "preferable" for song accompaniment - it depends entirely on the song. I'm half of a duo that has many instruments (not including a guitar, though) and we usually find for every song there's one instrumental arrangement that works perfectly and others that quite clearly don't. We also find that an English and Anglo concertina have quite different inherent song accompaniment styles, and that between 5 different melodeons there are good reasons for choosing one over the others for a particular song, and not just because of key.

It's certainly true that the choice of instrument can make a huge difference to the whole style of a song performance.

I suspect that a guitar in good hands has a very wide range of styles available. That would makes it a good choice if you want to use just one instrument, and may account for its popularity.

By the way, I've always thought that concertina *and* guitar together is a nice instrumental combination. Almost as good as cello and banjo...

Anahata