I've got both.
I bought the N900 outright. It's not compatible with the Telstra NextG (850mhz) network and falls back to 3g, however I love it and it;s my daily use device for email, general browsing and music etc.

The Desire was a freebie on my plan when the current contract expired and is a completely different device.
the desire is a Fantastic consumer phone and has everything most people would need in a converged/smart device.
Both devices have a range of issues that can be frustrating to users but at least they produce relatively frequent updates (if you want to learn the meaning of the word "wait" when it comes to updates, buy yourself a SE device) to address them.)

I'm no high end user of either device. I don't hack or code and neither have been rooted. I haven;t had any real issues with either phone beyond a few small glitches than one can expect from any device that gets rushed out to an impatient market.....