While the Zip does indeed have a larger, color screen it is extremely low-res and pixelated. It’s virtually worthless for seeing any album art, and even sometimes hard to read the text on it.
And no, it does not have any higher sound quality. The clips themselves are very similar; I doubt if one is any stronger than the other. I personally, also prefer the button pad on the Clip+ over the Zip. As the buttons extend all the way to the side of the casing on the Zip, it’s just too easy to accidentally press either the FF & REW button while simply grabbing it to remove it from your belt, pants, or whatever it is clipped too.
The Zip does have a couple of nice features that the Plus model does not though. One of which being the RDS feature on the FM Radio that displays (if the station is broadcasting it) the station call letters and track info on the song playing. That’s nice . . . if you happen to listen to the radio a lot . . . and if the station does its part.
Not really an obvious choice at all. Different people have different preferences and priorities.