I have been using the SII since July 2011. It is a fantastic phone and i still find it hard to let it go though i want to have a small "affair" with the SIII. First, the battery performance is excellent. If you want a proper review, check out GSM arena. There is a reason that phone has been high in the ratings for close to a year now!
The battery is 1650mAH. The longest my batter has lasted is 4.5 days with minimal browsing, calls,sms, some games and of course use of the camera. (I could post a screen pic of the hours spent on the battery). Constant use of GPS, hotspot function,bluetooth and/or games will suck up your batt juice in less than a day.
You can still buy a bigger battery (2250mAH) is you want to extend your batt life. However your phone will gain an extra 1mm in thickness. The SII is 8.5mm thick while SIII is 8.6mm.

I bought my SII in Eldoret phonexpress at 60K...now the SIII is about the same amount in all safcom shops. My opinion is, just buy the SIII. Fork out the 60K and enjoy the power of the "beast". Its got a quad core processor (1.4GHZ) and only phone that is in its league is the HTC one X. It looks fantastic in white and every time i look at it, i want to "stray" a bit. I want to have a small affair with it and still keep the SII. :)