I have the N9, and my (ex) gf has the Galaxy S2. My personal opinion is that it depends what you are going to use it for, and what you are looking for.

Things going for the N9:
- damn sexy
- proper multitasking
- (IMO) better looking screen
- build quality

Things going for the GS2:
- apps
- the screen feels easier to use
- swype (soon to change)
- you can access the battery!

The best thing to do, is to do tons of searches for reviews on both phones, then make an informed decision. each person is different.