Screen size is only marginal bigger, like 0.15" larger, except for the higher res. Both are dualcore, the Nexus cpu and gpu are better, but for practical usage as of now its only for bragging rights, you won't die from having to content with the cpu in the GSII.
Bigger battery yes, but from review and XDA impressions, people been complaining its shorter lived than the GSII.
My personal impressions from the photos that has been posted is the GSII takes nicer shots by a marginal amount and to me in this case, more MP does means something.
Also it have some RF shielding problems that effects the international GSM version of the phone, causing things like volume going to silence which causes missed calls. The problem persists even in its bootloader, which means its a hardware issue.
I am actually interested in the Nexus too, but two things needs to happen before I can put my GSII on ebay: Price drop, can't pay $800 for a phone. And for Samsung to fix the RF shielding problem, which mean one would need to wait for a revision batch (I am a bit paranoid I admit so I can see how this issue can be used as privacy exploit, what with the iPhone having capability to keylog what you type if put it next to your keyboard, and Google claiming to develop feature for phones to take in all kind of environmental queues).
The volume problem has been explained and is repairabke via software.