From a UI perspective, Rap Genius explains lyrics line-by-line whereas SongMeanings is more of a discussion of the whole song, and Rap Genius tends to canonicalize explanations whereas SongMeanings is more of a free-for-all discussion. The line-by-line system is much better for rap music.
For all music. You can select lines or stanzas, so it has all levels of scope in regard to the annotations.