Ok the Z70 weighs a pound less than the Z100 (due to its carbon fork) I assume and its also a 30 speed as opposed to a 24 speed.
Are there other differences in the components because you pay almost $400 more for the Z70..
I looked up your Trek sweet bike.. me in this position.. I do one of two things. I get one of the bikes in my post about bikes around $750 (I am shopping for such a bike and believe I'm getting a Lemond Etape). Or I take the leap and get something sick that is carbon fiber like the Schwinn Peloton.