EDIT :
Clip zip in short :
Screen has colors and more lines but lacks accuracy (not surprising since it is 96x96 vs 128x64). Some themes are actually unreadable even with a microscope. I guess they were made using the simulator !!
RDS works well & Radio tuning sounds better.
For the record my present Theme combination is =
runner_96x96.fms + zipplus.wps + Classic Clip UI.wps + clear backdrop (black.bmp)
Usuful tip : (theme bug) If you get stuck with a black screen in wps & sbs after changing the theme (happened to me with one), do this :
> connect through usb > edit /.rockbox/config.cfg = delete wps, sbs (fms?) settings > reboot (read the manual if you don't understand these short instructions)
Clip+ in short :
Screen brighter and MUCH more accurate (128) but lacks more lines (64).
Radio tuning sounds weaker and lacks RDS (ClipZip RDS actually helped me refine Clip+ presets

)
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Just got a new zip, while having used the + for a couple of years or so.
Not much to say !! Main difference is the screen. Battery life may be another.
I won't comment the battery life : I buy "refurbished" Sansa's so data could be inaccurate. "Could" because all players I got so far looked actually brand new : only they had no box, manual, etc...(zip was definitely unused : could not have been with opaque screen protection)
About the screen : geeks reviews of the zip have been keeping me away from the zip. I understood the screen resolution was so poor, it was useless for album art (which does not raise my interest any way) and actually not as good as Clip+. Also the zip was pricier...
Only when my sight started to go down (getting old) and I started fiddling with wps and theme AND price dropped, I thought I might give it a go.
Well that screen is... like they said... not great !... but not so crappy. It will be good for those who (still) enjoy a good sight (lucky guys...) as it allows to fit much more tags or whatever you like on screen. For guys like me it could be an improvement too because colors can be changed at will which should help devising a taylor made theme with readable, color identifiable tags and stuff. So far, no theme uploaded matches my taste (or rather my needs) but hopefully someone will post one... before I do (no hope before next summer if you're interested...)
Clip+ remains a good option, as far as I can see (haven't tried outside) screen is way brighter and more accurate, overall neater. What did you expect ? I wouldn't dump it as soon as I got it's younger sibbling !
[Teebark] posted a Screen photo of Clip Plus and Zip side by side :
http://forums.rockbox.org/index.php/topic,42558.msg216505.html#msg216505I wish that helps.