I couldn't decide on the iPhone 4S and the Galaxy Nexus or the S2 -- I ended up getting the iPhone 4S for a couple main reasons. And note, I do have the iPHone 4S but I was in the same situation right before I bought either phone. I previously owned an Android device and I still somewhat prefer an Android device for some reasons (mainly from the cons found below)
1. Apple apps are more polished -- Apps work better, look better and so on. There are definitely a lot more apps than Android, specifically games...
2. The iOS is more fluid than Android. Everything works as it should. Not that Android is BAD -- just it's not as fluid. Things don't work on Android as they should sometimes, even with the power some Android devices have. In iOS, it might be simple... and boring, but you're not going to run into any issues transitioning between apps, scrolling between them, opening them up, or running any other apps. The only thing that really stands Android over iOS apart is that you can personalize android a lot more heavily than you can iOS.... jailbreaking somewhat resolves this to a certain extent. Also, typing and clicking links just seems to work better.... see detail in screen
3. The camera is the best one out there on any phone right now. There is no arguing that it isn't. You can watch youtube videos, you can read a lot of the reviews. There is one review where it states the Galaxy S2 is better, but I think most user reviews from experience or other reviews state that it isn't. Not that S2's not bad at all. Specifically the indepth review that stated the iPhone 4S isn't the best mobile phone camera also made the situations where you weren't going to tell much of a difference anyways.
Lighting and color accuracy is much better on the iPhone -- pictures are sharper and more vibrant, it's one of the few phones where you can basically replace your point and shoot. You also can't exclude the shutter speed of the phone. It's not as quick as the Galaxy Nexus, but I think the quality + the speed definitely out does the Nexus.
4. Siri -- I'm sorry, I didn't think I'd use this feature much, I was like "oh, I'll rather us my planner at work, or use my calendar on my computer" I synced everything to my device and now use Siri to do all my calendars, reminders, notes and it's actually a big help... it's not perfect, but way more perfect than the other Android siri like apps out there. I'm often forgetting the most simplest stuff for tasks when I get home, the geo-tagging such as "Make sure I close out the process sheets first thing at work tomorrow" it sets up a reminder at right when I get to work...
5. The screen: I had a 4.3'' screen and I thought I'd have a big issue getting to the 3.5'' screen on the iPhone 4S and it turned out that I am starting to like the 4S's size over the 4.3'' that I previously had. Because of the fluid iOS -- I can actually type faster on the iPhone 4S.... so much for it being easier
Now the CONS"
Apps: Yeah, it's in the pro's. While apps are better than Androids as far as polished and feel. A lot of the paid apps are free-equivalents. So the choice is yours. I don't like how the market is setup for iOS and I miss it for Android. You could download multiple apps on android and still stay in the market. You can't do this from the appple market. I miss that.
Screen: I do miss the 4.3'' screen I had -- but it just didn't feel as comfortable as the 3.5'' on the iPhone.
Navigation: I really miss google navigation built right into your phone. The iOS navigation apps suck in my opinion, they do the job just fine, but I just don't like the 3rd party feel.
Customization: The biggest draw back and what I really miss is the ability to personalize your phone. I'm not about to download an app that makes borders for you icons...... and that's about it.
Overall both phones are great, you'll be happy with either or. I opted for the 4S because of the better camera, it's plenty fast, it's fluid, it's also a world phone which helps since I'm in the military -- there just isn't much flaws in iPhones. It might not be the best choice to power tinkering people like most people on these forums... but it offers a solid experience to most people out there, even the power users.