I owned both, for a short time. Pros and cons:
- S9 SQ is superior, due to the EQ / BBE settings. When played flat, the ZuneHD and the S9 sound similar (to my ears).
- ZuneHD has the better interface, out of the box. But, you can theme the S9 to your liking, which is a plus.
- I can play my Xvid / AC3 AVI's on my S9 without any transcoding (even if the rez is higher than 480x272). The ZuneHD didn't support Xvid last year at all, but I believe that's changed or will change with a firmware upgrade.
- Zune Pass is a great subscription service, if you are into that "all you can eat" mentality.
- S9 can do MSC or MTP, whichever you prefer. ZuneHD needs the Zune software.
- S9 works on non-Windows systems.
- ZuneHD has Wifi, a browser and an App Store which should hopefully grow over time.
- S9 supports more audio codecs, including FLAC.
- S9 has tactile buttons for volume, pause, FF/RW. ZuneHD does not.