Depends on your priorities.
If you want the best sounding MP3 player on the market, get the X5L.
I had a Nano and was not thrilled with the sound it put out. The equalizer settings were also inadequate and when adjusted up, the sound would have some harmonic distortion.
I got rid of the Nano and bought an X5L 20Gb. Its hands down the best MP3 player I've ever used. Turn on the BBE, MP3 enhance, Mach3 Bass to 7, Mach Treble to 10 and tailor the equalizer to the way you like it and it sounds stunning on my headphones.
For the record, I'm not an iPod hater or anything and there are a lot of things I dislike about the X5L such as the time it takes to skip songs, the interface and the occasional crashing. However, sound quality is most important to me and I'm more than thrilled and can live with its problems.