If an artist has a single you want to buy on iTunes and the song is available on both the album and a single album which one do you get if they are both the same? The one off the album or the one on the single album.
Generally, I would buy the album version if I already have some of the songs from that album, but if I think that I am never going to buy anything from that album again, I would get the single, because the artwork would be more revelent.