I tried to answer the same question for me a vew weeks back, and now I'm playing with my SGS2 (beside my loved N900)

+ Screen is amazing (just get the SuperAmolded+, not the SuperClearLCD !)
+ Speed is stunning, with no doubt much faster than N900
+ Camera is WAY better than N900 (and way better than SGS1's)
+ 1080p recording (looks great on my 65" Plasma)
+ MHL (HDMI out with USB)
+ USB OTG
+ "Billions" of Apps (I really appreciate the hard work of the community here, but why is it ONLY the Community supporting the N900 ???). Nokia has to proof that they will support Harmattan/Meego, I doubt it
+ Free navigation included (Nokia was promising it for N900 in my Country, but nothing happened 'til today)
+ Google-Sync included (Contacts, Calendar, Tasks, Picasa, Gmail, Google+, ...) - never loose my Data again
+ Front Facing Camera supportet (Videocall, Skype (Skype only via the XDA-Hack at the moment, because SGS2 is supportet since last Skype-Update but not enabled at the moment))
+ many other features onboard wich you can use with N900 (N9 ?) only with the help of the community
+ Updates (!), Android 4.0 is on the way (Gingerbread + Honeycomb = Ice Cream Sandwich)
+ Call-Log much better than N900's (overall list and individual contacts, including SMS ...)
+ Updates work much better (faster) - just decide auto/manually Update for your Apps, and there is absolutely no lag like on N900 where you have to wait for minutes after pushing the notification
+ Office Documents viewing AND editing from stock (Polaris Office)
+ superb browser with flash 10.3 support
+ very good Support-Forum (XDA), similar to TMO

-/+ no hardware keyboard was my biggest concern, but after a month of using the really good virtual-keyboard I'm faster than on the N900
-/+ no Camera Shutter, but it's Gorilla-Glass I found out, and also there will never again be a dusty lens
-/+ Akku has to be charged every evening (with Wifi/GPS/Skype/Weather App/RSS/Mail/Background-Sync always on, Browsing, Maps, Calling), but this is not much difference to my N900, from my feeling the N900 has given up earlier

- Wifi-quality lacks a little bit (N900 finds more networks and has better signal) - maybe the next update will solve this "problem" (not really a problem at home with strong signal)
- only one Speaker at the backside means no stereo-quality, and if you place your phone on the couch backside down you will hear nothing
- I had some unexpected Reboots so far, and still did't exactly find out what causes them (something to do with Samsung TouchWiz and the Stock-Apps), but decided to wait for the first big update before rooting and freezing/deleting all the crappy Stock-Apps


The SGS2 has been crowned as the best Android-Device so far from Engadget and other Sites:
http://www.engadget.com/2011/04/28/s...y-s-ii-review/

If you're not in hurry, I would wait for the first hands-on's on N9 and the still rumored Update of SGS2/SGS3 in fall

My decition was clear because I was very impressed by N900's hardware, but also very dissapointet by Nokia's epic fail in support and their broken promises (once again, after my N95 - the first Nokia device with GPS, was the one wich didn't get the free Ovimaps). May Elop rot in hell for eternity

@ Prozac 786

You didn't get the way how multitasking works on Android, did you ? There is NO need to close Apps, NO need for Taskkiller's, Android will close Apps by Priority when the System runs out of memory. The Home-Butten is equivalent to X-Button on N900, but there are also many Apps you can close completely by "Menu" -> "Exit"/"Quit"/"End"

For closing Text-Boxes in Browser just click the Back-Button

CU - M_99