Cell Phones and MP3 players now come with many more features than required for basic functionality. You don't need a camera to make a phone call, but your cell phone probably has a camera. You don't need a calculator to listen to music, but many MP3 players include a calculator.
I'm often asked what a player is worth, and find it a very hard question to answer. Many companies add unique features to their players to make them stand out from the crowd. You can do a simple comparison between two players by just counting features. If a feature has little real worth though (like a calendar you can't add appointments to), should it be given the same weight as an amazingly useful feature (like an email client that provides access to all your email accounts)? You may also discover two players with a feature, but only one with a version that is useful. The thorough calculator on my iPod Touch is FAR more useful than the basic calc (no mem, sq.rt., percent, +/-, 1/x, etc.) that comes on many of my Korean made players. You can't score both players the same for having a calculator.
Another consideration is "worth to you". A feature that is worthless to me may be incredibly useful to you. I don't care for any of the "social" apps out there, yet there are millions of FaceBook and Twitter and Linked-In members out there. Someone must find value in them. FM radio is common on many MP3 players. Some add RDS to it. The Zune HD allows listening to HD radio (if you can find any HD broadcasts in your neighborhood). Several player let you record off of FM radio, but they vary in what format and bitrate level (quality) is used. A few players will let you assign names (station IDs) to the preset stations by adding those names in a text file stored on the device. FM radio is very useful IMO, as local news, sports, and weather are often only available via local radio. Those extra features are where measuring gets tricky though. The Radio app may also only be used 2% of the total time the player is used. Should the features and quality of the radio app be compared equally with the features and quality of the Music app that may be used 85% of the time?
I could make a spreadsheet of course, and give each application a weight and sliding usefulness scale. I could grade each device on a point scale, with higher scores being better buys for the money being asked for them. It means some $20 players might get a higher score than some $200 players. It demands that price be considered, and that features have some monetary value to them. "Worth to you" is a variable I can't predict or control. It has to be stripped from the equation. That leaves a sterile result that you'll have to add your own relative value to.
The ability to play an MP3 file might be worth a standard 20 points, with how well it sounds to me reducing that to an A, B, C, or D grade. Or I could simply multiply 20 times the Signal/Noise ratio. The S/N number does usually relate to the quality of sound produced. The Samsung P3 has an 89 dB rating. The Cowon S9 has a 95 dB rating. I also think the S9 sounds better to me. By S/N rating the Samsung would get 89% of 20 points (17.8) compared to the Cowon's 95% of 20 points (19) for music playback value. Of course each player has other music properties, and they would be factored in as well. You can see how the math might get complicated.
Do you want a more detailed analysis of a players worth, or is a subjective grade enough?