I actually really love the new look of the Z, I'm not such a fan of the Desire HD though, as I was kinda hoping for a proper aluminium unibody design like the legend. I think it does look a little better than the Desire though.
Spec wise the Z looks pretty nice, and although the processor is clocked at only 800MHz the new 45nm architecture should mean a slight increase in performance and power consumption over the current Desire (as I understand it). But, if the speculations for gingerbread are to be believed, devices will need a minimum clock speed of 1GHz to run the new version, something that hobbles the Z is a huge way (if it turns out to be true!)
Other than that it's pretty similar to the Desire (screen size, RAM etc.) But with Qualcomm working on dual core CPU's it really won't be long before we're seeing mobile devices really blossoming, providing the battery technology increases in line with the processor enhancements to run the damn things!
Also, the Z has a real keyboard, something that may be a deal breaker for you if you value that kinda thing.