Pandora Radio, Spotify, iHeartRadio and Slacker Radio are in a tight race for your music dollar. All have evolved beyond their origins, offering more flexibility and additional features for your listening pleasure. Which to pick? See all four of these online music services compared here.

While Pandora holds the line as a pure music discovery service, our three other contenders offer additional services. For instance, Slacker and iHeartRadio have live-streaming terrestrial radio stations, and Spotify offers an enormous music library and the ability to share playlists.

Pandora has by far the biggest user base (65.6 million), while Slacker trails with only 4 million. But Slacker — which launched a redesign on Wednesday, boasts hand-picked song matches instead of those based on an algorithm. Not to mention it has nearly 10 times the number of songs Pandora has in its library.

Then there's Spotify, eight years younger than Pandora. It has the largest song library of the whole group (20 million) and still brings in an impressive 20 million users.

In the competitive market of online music streaming and discovery apps, which service will win this hotly contested battle? That's hard to say at this point, but the biggest winners in this competition will be music listeners.

Which one is the best? It depends on features what you're looking for, such as custom radio stations, paid tiers and song selection. Take a look at the table below, and you'll get a good idea of what's offered by each service, and then you can compare them directly with each other.

Let us know in the comments which music service you think is the best, and why.

Pandora vs. Spotify vs. Slacker vs. iHeartRadio

Pandora Spotify Slacker iHeartRadio Monthly Unique Users 65.6 million 20 million 4 million 48 million Launched 2000 2008 2007 2008 Type Discovery Discovery/on-demand Discovery/on-demand Discovery/live radio streaming Create Custom Stations? Yes Yes Yes Yes Free version available Yes: limited skips, ads, reduced quality Yes: ads, limited listening time Yes: ads, limited skipping Yes: fifteen combined skips per day, six per hour per station Paid Tiers PandoraOne: $36/year or $3.99/month $4.99/mo. for desktop & laptop, no ads. $9.99/month lets you use all devices, no ads $3.99 ad-free, $9.99 custom playlists and full on-demand streaming No paid tiers Track Rating Yes Yes Yes Yes Music Library Size 900,000 20 million 13 million 15 million Stations Custom stations, supported by Pandora algorithm Custom, unlimited skipping Custom, 200+ curated stations, 750 terrestrial stations, live talk radio Custom, 1,500 Live Stations Social Media Integration Yes Yes Yes Yes Mobile Apps iOS, Android, Windows Phone, BlackBerry iOS, Android, Windows Phone, BlackBerry, S60 (Symbian), Windows Mobile iOS, Android, Windows Phone, BlackBerry iOS, Android, Windows Phone, BlackBerry

Graphics courtesy Pandora, Spotify, Slacker and iHeartMusic; composite by Mashable/Charlie White