Essentially, the choice between these two is simple. If you want a shiny snazzy neat smartphone, get an HD2 (or consider an HTC Desire, smaller screen than the HD2 but runs android rather than winmo and a newer Sense UI), if you want a pocketable tablet that also happens to have (very basic) phone functionality, get an N900.
I own an N900 and my girlfriend has an HD2 so I have pretty extensive use with both, and theyre both really neat devices in their own right but are frankly aimed at different markets.

If you do choose the smartphone route though, I would give strong consideration to the Desire rather than the HD2. While the fact that the HD2 runs Winmo isnt really a major issue in my opinion (the platform gets much more stick than it deserves, especially with HTC's Sense UI glossing it up), non-business app availability for Winmo is tiny compared to Android, so you're going to end up with a device that can do a lot more consumer-oriented stuff if you go for a Desire instead of an HD2.

Thanks, guess I'll go HD2 after all, at least I will be certain it will have few years of active support from the company..

And with the Desire.. I'd go for it, if it didn't run Android.

Well maybe you should get the n900. Every other week something new and impressive is happening. A lot of people are selling them but they don't know. Why not brush up on your linux skills and play with one.

You will be impressed and as far as community goes they still have a community for the Tablet 770, 800, and 810. All those devices are considered way past their prime and people are still doing things for those particular platforms. And since the n900 will support Qt all the new higher-end devices will all be able to run the same app. I think that all the cool things that Nokia is doing with phones lately, the n900 is the prototype device that is used. Like the technology that allows you to download a whole album at the Nokia store in like 8 secs.

The HD2 is like one year past its prime. It if came out last year, go for it. Now you know that phone will not be getting Windows 7 and they don't have a lot of apps that you would find useful as a user. I am pretty sure it will be hacked to death but any program that you find useful on another platform it already exists in the Maemo community is some shape. And if it doesn't you always have the ability to go to the website itself and do what you need to do.

Just my two cents.

Oh, I missed the news that HD2 won't get Win7..
Well, that's one more point for N900..
..how hard exactly is it to run applications that are in development there? There are lots of cool stuff on forums I'd like, but I don't really want to go all shell on the phone..

Just add the devel (development) catalogue and u can run most (90%) of the apps that are in development.
I would really choose the N900 over the HD2 (I thought about getting the HD2 first too). But both are great phones for sure.

Do I need to download the apps straight to phone, or can I download them to PC and then into the phone? Cause, I won't have any data plan..

..also, another thing I'm afraid of with N900 is the stylus.. I seriously hate styluses.. how much do I need to use it?

Very rarely, i love to use the stylus in small areas like CorsixTH and OpenTTD, thats about it!

I used a HD2 today, it was the least user friendly and intuitive phone i have used!

If you want a phone then don't get the N900,
If you want an internet tablet then get the N900

Simples...

The problem is that when we look for a cell phone, we have different priorities .., some look for multimedia performance (for them perfect iphone,) other games .. (WM platforms and even the iPhone), other office automation (android, blackberry), others only telephony and a bit of everything (Symbian sony ericsson, motorola) ... but we who believe the perfect mobile PC is an extension of more if it is based on Linux .., we have developers soul (single soul, then we body missing) are gut-all geeks, experience and marvel not seeing the Mac-OS running on a screen of 3.5. want to live permanently connected to the Web (with full flash support and other protocols), to social networks, etc ... .. for these .., and I include myself in them ... the N900 is a godsend .., and even used to speak as by phone ..

I guess I'll probably get the N900 after all.. we'll see after I will get my hands on it..
Just another dumb question.. do I understand it correctly, that after the update most of the built-in apps support portrait mode? I will be probably using landscape mode more often, but for simple tasks that I will want to be performed fast as checking out time/new sms/etc, I just can't imagine going landscape..

Also, are the updates only over the air, or can I update it with PC too? And I can update it freely, without any region/operator issues, right?.)