Nexus
Plus:
Jelly Bean now
OS updates very timely
Carrier Independence
No vendor skin applied to stock Jelly Bean experience (Touch-Wiz)
Minus:
Not so great camera
Not so great processor
No LTE - Ever
No expandability (no MicroSD card slot)
Galaxy SII
Plus:
Better processor
Better screen (I hate saying this because it is so subjective)
LTE (if your area supports it and it is expanding rapidly in AT&T land)
MicroSD card capability for expandability
Minus:
ICS and probably 5 months before Jelly Bean
Touch-Wiz
Bloatware (Vendor and Carrier add on apps)
If you are the impatient type who can't wait for Jelly Bean then Nexus is your choice unless you want to root and custom ROM the brand new SII. The reason I am going with SII is many of the minuses of the Nexus are permanent (no LTE, no MicroSD, 5Megapixel camera) while many of the major minuses of the SII are fixable (no Jelly Bean, Touch-Wiz) with patience and a third party launcher.
Just my take but both would be major steps up coming from a BB9800...
Mike