OP, I am using an international version of SII and have recently upgraded to ICS.
To be honest, if you are not a phone crazy, the SII is very much good enough. And if you compare the Nexus and the SII, I will
strongly suggest you the SII!!
The dude work next door to me has recently got the Nexus, it has a bigger display, that's it, oh... it comes with lagging too.
One time I put his phone side-by-side with the SII and right away I can see the SII display is much brighter and
white color is displayed in white. The SII actually does have a better hardware display screen than the Nexus.
Web page loading is about the same but I'll say the SII is a bit faster, both on same network, but page scrolling with the Nexus is laaaaaggggggginnnnnnggggg....
There was quite some silent from that dude after we did the camparo....
Consider his phone is new and have limited apps preload and mine is 8 months with lots of preload apps.... you know what I mean...
About the ICS OS, it adds some fancy gesture, like face unlock(which I'll never use), new keyboard(which you have to get used to), slightly better battery performance and WiFi is a tad better. Does it make the phone run quicker?? Not really... but it a no-harm upgrade.
As I said, if you are looking for a phone for e-mail, internet broswing, words and stuffs like that, the SII is very much good enough, cos this is what I mainly use my phone for, I don't really play games with it.
For the SIII, it says the launch day is in May, but it's not firm yet, and even if it really launches in May(which I highly doubt), it will be in Europe, then Asia, then it will take at least another 3-5 months to come to N.A. carrier, because each carrier will have to make their own OS for the phone before they launch it, so you will have to wait til at least Aug or further.
I will not bother to go for the Nexus, if you are looking at phones on the current market, then I will strongly suggest the SII, either upgrade to ICS or not, this phone is more than everything. The camera is extremely good too, I never have to use flash and the pictures come out nicely.
Since the first generation of Palm-Phone, this is my 8th smartphone, I still have my HTC Touch HD and HDII in my draw, LOL.
If you ask "Should I wait for the SIII?", I will say the SIII does worth to wait for because it's a freaking good phone, the quad-core 1.5Ghz(maybe 1.8Ghz) processor with the 16G RAM and SD slot support up to another 32G, plus the 12MP camera, app 312ppi display and Ver.4 bluetooth..... To be honest, I am eyeing this phone for myself too
Sorry for the long post but I hope this is helpful for you.
cheers