Galaxy Nexus. No offense to current GSII users, but your phones are last-gen low def devices now.
I'd take the (unproven) hit in performance with the speed-bumped PowerVR SGX540 over having a low-definition device ANY day of the week.
The screen on the GS2 is absolutely amazing, looks sharp enough to rival the 720p pentile display that the nexus possesses.
Comparing it to my captivate screen, it was not even close. My screen has higher pixel density (4 vs 4.3) but the sgs2 was so sharp, i would have guessed it was a higher res if i didnt already know.
That paired with the fact that higher res screens wont be the norm for another year. and how is that super high res screen going to be able to power later versions of android with a less powerful cpu.
I think im going SGS2 on this one.
I really only have one concern: If the sgs2 will get REAL support, and have good CM based roms.
I currently have the captivate (which is offically supported by CM) but because they dont have many drivers, the rom has some bugs/battery issues. Theyve reached a point where the bugs are pretty much all gone, but the battery is atrocious. (i get 10-12 hours on CM with moderate use vs 24+ on a captivate based rom.)
If this is the case with the gs2, then id lean towards the nexus. Im 100% sure it will run CM better than any other device.
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Im 99% sure i want to pick up a GS2 for free on black friday, wheres a place that im guaranteed to get one. I dont mind waiting in line, but as long as i can definitely get one somewhere for that price it doesn't matter.