I chose Napster because of the name recognition (OK, so it doesn't really matter, but its good to have the oldest, most experienced players backing up the service), ease of use and flexible pricing.

In addition, Roxio's (they bought Napster) software is top-notch.

My suggestion would be to try each in turn for a month, then choose the one you like. My suspicion is that there will be price wars, and many of the players entering the market will go out of it just as quickly. Roxio/Napster should be around about as long as any of them.

Good luck!