Unless you apply Cowon's good SFX and EQ, you aren't really getting anything different from even a regular iPod with Cowon. That said, if you use earphones, the Cowon players generally begin missing beats: losing resolution in bass or treble, narrowing stereo separation pretty low, and other things. They still sound good for most music, but they can get congested.
The S:Flo2, while touting a rubbish interface and pretty worthless touchscreen, is unaffected by headphones. Plug anything in, even the lowest Ω headphone and it will sound the same or very similar as the unloaded signal. The S:Flo has the most impressively strong headphone out I've used. I am not talking about volume, but about quality. That said, it has a little hiss (about the same as an S9), a constant small ticking in the background, and is a person to update with new music.
If I had to choose between it and the S9 or J3 for SQ alone, I'd take it hands down. But for a music player where I could enjoy what I am listening to, I'd go Cowon.