For those on the fence and switch all the time- the best solution is to get one of each- one android, one iOS device. iPad/GS3, iPhone/Transformer Prime, whatever.
The bottom line is that ICS is not going to be as smooth or as reliable (think pulling the battery, freezes now and then) as iOS. Jelly Bean could be a different story. BUT don't delude yourself into thinking, "Well I'm gonna get the GS3, wait for the Jelly Bean update and Christmas will come early for me." You will really hate Android then. You should think that your OS version is as permanent and fixed as the processor in your Android phone. And if you want to upgrade, you need to buy a new phone. Really. The waiting for the update will kill you.
Obviously, if you plan to root and install a custom Jelly Bean ROM, then disregard everything I just said. But for those users who don't want anything to root, remember, OS version=processor in terms of upgradability. I'm not saying it's impossible to the the OS update, but you've got the same chances of getting Jelly Bean as you have of winning the lotto.
So if you are considering the GS3 or any Android phone, evaluate it as if there is no new OS upgrade and that you will never get a major OS upgrade. If you love the GS3 as is, ICS and all, then definitely buy the phone. If you love the GS3, but really want Jelly Bean and don't want to root, then wait for a JB phone.