I have the at&t version of the SII, which imo is the best of those models. It's size is perfect with the smaller 4.3" screen and it's smooth and fast with the Exynos dual core. I've always had iPhones and never cared for Droids before until using this one, love it. I cancelled my order on the 4S, nothing against the 4S, it's a great phone, I just preferred this one. Screen is gorgeous and can be tweaked even further with screen adjuster app like I used, just so much nicer and easier to browser on vs my smaller iPhone screens, especially at night time if I kick back in bed.... can read the text/browse at a further more comfortable distance (and my eye sight is beyond excellent at 20/15).
Battery life is great too, once you calibrate it (just like i had to calibrate my Macbook pro so the charge % is read accurately). I'm getting similar battery life as my iPhone 4 in use though the 4 still has better standby time, and better then the 3GS or 4S. Androids are more sensitive to battery life, you have to know how to setup the phone correctly, and setup app's that may be checking online constantly to download info or constantly run etc.
I'm still going to flash with uNnamed rom to get rid of the AT&T bloatware, and you get even better battery life along with all the custom tweaks of the Rom. But the phone is pretty sweet, very light and is actually more comfortable to use as it's not that much bigger then an iPhone.
Can't go wrong either way (4S or SGII) both phones are top notch, just depends on which you'd prefer especially if your really intertwined with Apples app store or ecosystem heavily... which though I have other Apple products, doesn't mean much to me. Plus I travel a decent amount either for vaycay or biz, and love how integrated Google Maps is with the Nav system, I've used a few others on the iPhone and soooo wish Google released an app like that for Apple, just the integration and especially how fast it can re-route is sweet.
Oh yeh - Swype kicks ass lol, I didn't care for it at first, but once you get used to it after like a day or two and it starts learning your "typing/words"... it's so slick to use.
