The 6600 does retail at $1999, so its $250 less than a Z1, but the Z1 does do a couple things the 6600 doesn't.
The 6600 has a 7 band parametric EQ, the Z1 only a 3 band. Both have XOVER's built in, and some sort of time alignment (nothing extensive, more like a listening position selector). The 6600 has three 5 volt pre outs, Z1 only has 2 volts. Both do Full iPod control. 6600 does Sirius, Z1 does XM and Sirius.
This is where the Z1 takes off over the 6600. Z1 has bluetooth built in for cell phone use, voice recognition for NAVI and audio (say next track and it goes to next track, something like that), and a hard drive built in for ripping music.
The 6600 has, in my opinion, a better audio section, the DAC's and features are better than the Z1, but the Z1 has more modern convenience features. In my opinion, the Z1 would sweet if paired with a nice external EQ, but then the price goes up.
The 6600 is definately a nice looking unit, and not bad for the price, its worth looking at atleast (Z1 is out, there should be some people popping up on the forums in the next couple weeks with opinions).