Every Tuesday, Rock Band releases three to five new, downloadable songs (that are compatible with both games). In addition, they may also release a track pack or album if they happen to release three or more songs per artist in a week or month. In these cases, not only can you download them as a whole, you can download the ala carte (separately). Typically, there not a lot of cost savings with the smaller 3 song packs, which average about 440 microsoft points (a 40 point discount from buying them individually; 160 points a piece), but you can save a lot more on larger packs and albums (in most cases).
In this article, I am not listing all of the downloads for each month, rather some of the top choices of what's available. If I missed something you think is worthwhile for that month, let me know and I'll gladly check it out.