The true question is : What do you want? You have to know that. Once you know that thing, put in one side the N900 and in the other hand the desire. Then, compare things that the n900 has and the desire not, and viceversa; then you can choose. About the GPS voice navigation, you can use Sygic. Yeah, is a pay program, but if you google properly, you will find a "discount". The n900 also lacks of multitouch...
by the way, i think that the htc desire is for "medium-heavy" users; it have a lot of cool thing but you can do anything with them. In the other side, you have the n900. I have to be honest; if you leave the n900 with the factory things/settings ,you will be dissapointed (at least, me) But in the moment that you install some apps (like kernel power for overclock, ipv6) and stuff like that, you will have a phone that is capable to do things that any phone can't do.
If I you, i choose N900, with no doubts.

just to add a little comment, sygic (10) really sucks, I have used it to travel, and honestly it's barely unusable when it comes to complex situations, with multiples roads, it's slow, the colours used are weird, it's unstable... It does it's job, but there are far better gps apps (not on N900 of course)

If you don't want any headache, go for a desire. If you like hacking or tweaking your phone at a low level, the N900 is the perfect phone for you.

Get the Desire HD (Not standard).

768MB ram and a 1ghz snapdragon 2nd gen (48mn core).

The N900 is the best phone for a true geek, but the DHD is a damn great phone!

I got the Nokia C6-01. It's beautiful! But the N900 still performs better in most areas.

What about the samsung s epic? it's got everthing the n900 has too ( no infrared, but multitouch ) and is running Android!

Can we collectively realize that Sygic isn't the only voice navigation solution available? There's at least two solid ones: Mappero, and Navit. I just installed Navit using an install script from the thread discussing the Navit port on the N900, it works really well. Both use eSpeak, which like most open source projects, is rather flexible. In Mappero, you can even replace the voice files used with your own easily, and as far as I know, you should be able to do the same to eSpeak as a whole, thus changing Navit's voice guidance. (In a quiet room eSpeak will sound rather loud, but when you're driving it won't be noticeably loud. The ONLY issue is that you have to work a little to download the maps, and the voice navigation volume from eSpeak, near as I can tell, does not change with your N900's volume settings. Either it's on in the General profile, or it's off in the silent. [I don't use more than the two stock profiles - never needed more.])

It used to be that I'd recommend Navit over Mappero because it had both voice AND an internal routing engine. HOWEVER, I just saw a thing called "MoNav routing daemon" in FApMan (in Devel only for now), which basically is just that - a routing engine, so that all those programs that needed to go online to route, can now be written to use MoNav instead.... NOT suggesting you download this RIGHT now, but since Navit isn't being as actively developed as Mappero, this is extremely promising, and I would wager in months at the latest, possibly in days, we'll see the 10-or-so available mapping/navigation programs on the N900 using it (since most already use Open Street Map project maps for their mapping).

I'm guessing 6-3 months ago, users of the N900 could turn to Sygic because nothing better did exist back then, and the open source stuff wasn't up to par. But I would say it is, by now.

Anyway, I won't type up a full opinion about advantages of a Desire or an N900, but for the maps themselves, you don't even have to pay anything at this point. Just install Mappero, or dig up the thread and install Navit, and you're set.

(Okay, I can understand how I type "not" instead of now [muscle memory for 'not' more ingrained then for 'now'], but how the hell do I manage to type "great" when I meant to type "thread"?)

i beg to disagree that resistive touchscreen is a downside or maybe it's by preference. i still prefer my n900's screen over my itouch 4g. editing and copy/paste is so painful on a capacitive. pinch might be better than swirl but putting just a bit more effort wouldn't hurt or even kill you. same as applying like 3-5% more pressure on the n900's resistive. it's sensitive...

a psp or ds for a video game is way better in playing. i'll stick with physical buttons than multi touch thank you very much...

n900's multitasking prowess is still the best...

if you can't wait, i'll go for n900 or desire z / g2. if you can wait, might as well wait for those dual core processors (i know lg already released on in korea) next year.

Shun the N900. It's an EOL (end of life) item and despite it being incredibly able; it does not have a lot of consumer level items - read: support, popular and modern games, gaming frameworks like OpenFeint for instance - and ultimately you're tied to this (albeit awesome) community, you'll not have items that will get upgrades like Skype for instance.

Shun the HTC Desire. The build quality, while decent, isn't great. Expand your options if you could. Good luck with your endeavors.

Having an n900 is like having a wife, needing daily loving attention.
Anything else is like a rent-a-girl. No long-term involvements necessary.

(sorry n900_ladies I am sure you can come up with something more eloquent)

N900 wins Desire, but there are better phones than N900 for you.

If your in USA:
I'd recommend the EPIC 4G.

If not in the USA (ie GSM frequency):
I'd recommend the Motorola Milestone2

If you cannot tolerate MotoBlur (and can't wait months for custom ROMs):
I'd recommend the HTC Desire Z

If you don't need QWERTY at all:
Get Nexus One (SLCD), its the only beast phone with a physical trackball (great for Quake 3, the like, apps and browser).

If you don't need that aswell:
Get the iPhone 4, or the Nokia N8, or the Acer Stream S110.
That's the short list, all other smartphones seem inferior to these options.

But this is assuming you need to buy it now and that you can't wait for the N9/MeeGo/iPhone5/killerDroid.