I'm using lumia 800 as my work phone and the n9 as my personal phone So I quess I can share a couple of thoughts...

IMO lumia beats the n9 in one thing: exchange integration. Would be quite funny if it didn't...

since windows phones don't have real multitasking, etc. it keeps the UI responsive while the n9's UI lags a bit every now and then. The responsivity of the windows phone comes with a big hit on usability. The UI design on the windows phone isn't really that good and the missing multitasking just makes the problem worse so usability/UI design points go to the N9 6 - 0.

There's a lot more applications available to windows phone but the amount of quality applications is about the same (for my use) with one big difference: there is no terminal or a good ssh client for windows phone (there's one decent 3€ ssh client application). For me this is a big thing and limits usage of the windows phone too much.

Both devices are missing one important thing: hardware keyboard.

For me N9 is the clear winner.