N9 and Lumia 800 are already in the market, I bet a lot of fans are trying to look for review or comparing both of the phone. Well, I'm not N9 or Lumia 800 owner, I'm N900 owner. I'll just compare it base on my knowledge or whatever I know.
First of all, N9 is running on a all new operating system which is Meego Harmattan, special for N9 and it's difference from the other meego, it work with swipe, buttonless, that's the special point of harmattan.
I tried to play with N9 in Nokia shop, It's pretty smooth.
Lumia 800, using Window mobile 7.5 platform, it's a new OS, but at least, before this there are some older version of windows mobile, their technology and development will be better than meego.
So my conclusion is,
If you're an ordinary handphone user, I suggest you to take Lumia 800,
If you're someone who like to explore, who like to try new things, you can try Nokia N9, it can flash to custom kernel, I not sure is there any custom kernel on the internet now, but phone with custom kernel you can do a lot of things, n900 with custom kernel can overclock, install android, install meego, kubuntu, backtrack5.
Before this I did posted Ubuntu Mobile Concept
Choose the right phone for yourself, if you like apps, like whatsapp or some other stuff, Window mobile will be your choice.