Be skeptical and demand proof by asking to listen for yourself. You might save yourself money. I think the iPhone 3GS and the Clip+ sound just as good on all sorts of headphones, if not better, than those Cowons and Sonys (even the X series). Moreover, if you turn off the EQ, the iPhone has better measured performance. For example, the Cowon players still have a blocking capacitor on their headphone amplifier's output stage, and that results in weak bass on low impedance 'phones. Apple fixed that problem on the iPods years ago. The Sansa Clip/Clip+/Fuze have lower output impedance, and that sometimes results in a more uniform frequency response. That's about the only technical spec where the Sansas best the iPhone 3GS. The Clip+ and the original Fuze are reasonable purchases--you can find them refurb for $25-$35, if you want backup.
If you need a flexible equalizer download EQu and Equalizer from the app store (total cost: $6). You might find one of them usable. For a real improvement in sound quality, spend the money on better headphones.