The first thing you should do is decide what features you want and what your budget is.
It looks like the Pioneer units support XM and Sirius, but the Eclipse only supports Sirius. I have used both, and it seemed to me like Sirius had more stations that I liked, but the XM stations seemed to be better. So, I personally didn't think one was better than the other.
The Z2 and 6600/6610 units support the satellite traffic updates (there is an extra monthly charge).
It looks like the Z2 has a hard drive. I guess you only load the Navigation DVD once, which leaves the DVD open for playing CD's, Movies, etc. Usually the hard drive models allow you to rip CS's, so you don't need a CD changer.
The main difference between the 5500 and 6500 series Eclipse is the satellite traffic updates and the dual DVD player. So with the 5500, you can use Navigation or watch a DVD movie, but you can't do both.
I think all of them have iPod support, which does me no good since I use a Zune player.
The big question would be which Navigation is the best. I am probably going to stick with the Eclipse, since I know I like it, but I have no idea how good the Pioneer is. I like the Eclipse better than my previous Nav system which was Alpine.
That's about all I know...
alaskanboi, I like the custom sub! I'm thinking about doing something similar.