The better option would be not to buy any of these units at all, no matter what the brand
I'm serious. First, get one of the newer, larger Android phones - The Sprint Evo, The Verizon Droid X, etc - one of the ones with the 4.3 inch hi-res screen. Or the new iPhone with it's slightly smaller screen, whatever, all that matters is you get something that has an interface you like and can work with.
Then get a nice car adapter dock for it - ProClips sells the best ones, I think, though they can run you $80 for the phone dock + attachment. They look really nice though, and the docks allow you to permanently hide the usb cable (for charging, or music) so you get a nice clean look and you just pop your phone in.
Once you've done that? Forgo a DVD/Nav/etc double din unit entirely. Buy a nice single-din Eclipse or Kenwood or Alpine or something, one that accepts USB/Bluetooth audio streaming. Focus on sound quality because it really doesn't matter about the display on it since you'll:
Use the display on the phone for everything. Seriously. The new Droid X, etc screens, are, when turned sideways (which the ProClip mount allows you do do, easily) not all that much smaller than the 6.1 inch screen of the 920bt. I mean, yeah, sure it's a almost a couple inches smaller, but the point is it's big enough to do what you need it to, which is control your music right from the phone, nicely. Or anything else you want, like Navigation - I don't know if you've seen the (free) Google turn by turn Navigation on these new phones, but it's miles above the Navteq crap the current crop of Pioneers use. Or you can get TomTom/Garmin for them if that's your thing.
That's how I'd do it if I could go back and do it over. My wife just got a Droid X and when she's in the car we just use that and stream audio over bluetooth, it's a much nicer interface. Pandora support, Last.fm even, whatever you want.
I know it sounds crazy because the point of the Z120Bt/920Bt/etc is to "integrate" everything but I've come to realize that you give up a lot just to get a little bigger screen built in to your dash, one that every thief in your neighborhood is going to target at some point. These new phones with their app support and now the bigger, higher resolution screens are better in every way. If you worry about looks or having cables hanging around then just get a ProClips dock and call it done.
What does a bigger screen get you anyway with the in dash units like the x920bt? Easier to see the nav maps? Great, I get a nice large picture of a perfectly buggered Navigation system. I'd rather go down a step in size to get a better system that has awesome POI support, customizable voices and voice control, built in (free) traffic reports that are better than the crappy MSN Direct reports that you actually have to pay for, etc etc. What else is the larger screen good for, music playback? The Droid X player shows everything you need to see in nice large text and the controls are actually bigger on it's smaller screen than the ones on the Pioneer. DVD playback? Please. First, how much are you (or should you) going to watch movies in the front seat of your car, while driving, which by the way is against the law in the US? Enough that you really are going to say "you know, this 4.3 inch screen is too small, 7 inches is just the ticket?" No. You want DVD playback in the back, get a DVD player for back there, you don't want to control that crap from the front seat anyway. Your kids will figure out the play and pause buttons soon enough.
As you can probably guess I am not a fan of this, my first, and last, double-din in dash Nav/DVD/iPod unit, the x920bt. Last week I got so sick of MusicSphere - a truly terrible application considering what it is supposed to do - that I just switched control of my iPhone from the x920BT to the iPhone itself. It's mounted in my own ProClip dock and you know what? I prefer controlling it from the phone, smaller screen or not. And I get my Genius list support, which is what MusicSphere tried to be and failed at, *hard*. And I can use the actual Pandora app and not the crappy PandoraLink mess which bugs out on me at least a third of the time I try to use it.
That's my suggestion - get a good car stereo without all the frills and let the new generation of Smartphones be your controller. By a good car dock like Proclips, mount it to where you can easily reach it - chances are it'll end up closer to you than the in-dash unit - and enjoy. Smaller screen or not it's the better alternative.
There's also the iPad, which could be a truly awesome alternative (ProClips even makes a car dock for it!) but currently is a bit limited as far as it's dock and GPS capabilities. I expect as that device matures in terms of hardware and software it will become a *very* popular Car accessory.