As a musician who's recorded a lot of CDs with unsigned bands, I'm a longtime fan of CD Baby, which provides an online store for selling physical CDs, as well as digital distribution through iTunes and other online services. How do the services compare for digital distribution?
CD Baby charges a one-time $35 fee for each album you want to sell through them (digital or physical), and takes a 9% cut of each download. TuneCore charges about the same amount up-front (although it will vary depending on how many songs are on an album and how many stores you want to sell your music through), and charges an additional $20 per album per year. But they never take a cut of any sales.
If you're a relative unknown with a local fanbase and minimal tours, you might sell 100 tracks in a year. From each of those downloads, you'll probably earn about $0.60 of the $0.99 that most sites, including iTunes, charge. Sixty bucks. CD Baby takes 9%, leaving you with $54. TuneCore's always going to be better in that first year, as they let you keep the full $60.
But in the 2nd year, you'll pay TuneCore $20 per album regardless of whether it sells or not. Or you'll pay CD Baby 9% of your gross from digital sales. Assuming the $0.60 per download is accurate, the breakeven point is 370 downloads: that is, you will have to sell more than 370 downloads to do better with TuneCore than with CD Baby. For a name like Public Enemy, that's a no-brainer. For other unsigned bands--depends on whether you tour, whether your MySpace friends actually translate into paying customers, and so on.
If you're signed with a label, they've probably already covered digital distribution for you, and the contract says what the contract says.