In my opinion, go ahead and buy the galaxy s2 coz i think its better than the galaxy nexus, in terms of camera, battery life and performance. Galaxy S2 with ICS gives better performance than the nexus.
Coming on having jelly bean, it is being anticipated that if Google does release jelly bean this year, it will be an upgrade to honeycomb that is jelly bean will be only for tablets. I am not 100% sure about this, but if you research more on the internet, a lot of websites say that it is likely to be just for android tablets.
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also i m pretty sure that if galaxy nexus gets the jelly bean update, the galaxy s2 will also most likely get it.