One thing to note about the Z1 and the 10GB for music. Music is ripped from a CD you put in to play, there is no way to just put a bunch of MP3s on the HD, nor is there a way to get the data off of it.
I think Eclipse's website is :
www.eclipse-web.com
Pioneer's is :
www.pioneerelectronics.com (or
www.drivehappier.com)
Someone correct me if I am wrong, but I believe both the Z1 and an Eclipse unit require the trim pieces to cover gaps left on either side. So which appears more stock I think comes down to button color and user opinion. (Z1's buttons are lit blue, so that doesn't match very well)
Some other differences:
Z1 can support both XM and Sirius (integrated menus for each), and has the option for NavTraffic.
Z1 has 11 million points-of-interest compared to the Eclipse's 6 million (DVD versions, the 5495 has 3.1 million POI)
Z1 and the Eclipse 5495 are HDD based, so NAV response should be a little quicker. Other Eclipse's are DVD based for NAV.
The 5500 and 6600 are HD-radio ready, and I do not believe the Z1 is.
Here is the link comparing the 5435, 5500 and 6600:
http://www.eclipse-web.com/avn7000/fea-list2.html