I would welcome you to N900 community, but why don't you just try flashing your Desire Z with the latest rom Icecream Sandwich before deciding to sell it?
Or you have already done everything on that phone and thought it is time to move on?
For everyday use: DZ is better
If you hack DZ: It's better still
If you will use the hacks (sudo ring any bells): The N900 is far better.
Basically get the N900 (only if you use it to its potential) otherwise get a better qwert-droid like the Captivate Glide, Droid4 or EPIC 4G (If you get CDMA).
Otherwise larger-droids like the Gnex and NOTE are sufficient enough to drop the physical keypad and slim down the device