I´ve got both.
Keep going back to the N900.
If you like to tweak your phone (themes, icons, put widgets and shortcuts WHEREVER you want) then the N900 is your choice.
I can also say that with equal use, the Desire has had more lock ups/freezes and application fails than my N900, however the phone app on the N900 is pretty shite.
Delays in unlocking the screen and having to unlock the phone just to end a call can all be too frequent and annoying.
The N900 email application is great ( I like the fact that I can have multiple accounts that I can see/select all from the same screen, unlike the Desire which you need to switch between)
If you want an internet device that that has a phone, get the N900.
If you want a phone with internet capabilities, get the Desire