In terms of specs the Z and the HTC Universal have edges over each other:
Pros for the Z: CF slot, internal disk space, better quality screen.
Pros for the Universal: a full range of connectivity options, SDIO (and higher SD interface?), keyboard w/ backlight, faster processor (PXA270@520MHz), dual-camera, microphone, Transflective TFT, more power options (up to 4800 or even 5200 mAh!)
Otherwise, their weight, form factor, dimensions, pocketability and now compatibility with linux are very similar.
Then again, personally the Universal has more cool factors and feels more geeky. The Z just seems to be getting older and older now...