I have 3 clip+ and 2 clipZip in my family, all rockboxed.
2 of the clip+ players, their power button has broken (switch broke off circuit board).
The Zip has an improved, wider, probably sturdier power button.
I would not buy another clip+ because of this.
Rockbox works fine on the clip-zip but the themes are sparse and immature, docs, etc.
Stuff you will miss on the clip-zip will be themes, backgrounds, manuals, etc.
Stuff you will miss on the clip-plus will be the ability to turn it on when the power button breaks.
(They are still sort of usable, inserting a usb cable turns it on, and who needs a stop button?)