Both phones have their positives and negatives.
The PenTile Super AMOLED HD vs. the Super AMOLED+ display is subjective. Some people like the higher resolution, some people would rather do without the green tint.
Both are NFC enabled, so that's a moot point, too.
About the processors: People in this thread seem to think that every single mobile SOC uses its own native ARM ISA. This is false. The TI OMAP 4460's processor and the processor in the Exynos (And for that matter, the Tegra 2's and the Apple A5's) are the same (Save for certain cases like NEON implementation, which most, but not all SOC's do. The Tegra 2 doesn't.) The Snapdragon's is not, as Qualcomm licenses the ability to design their own ARM based ISA from ARM Holdings (Think AMD and Intel).
http://www.anandtech.com/show/3632/anands-google-nexus-one-review/9
Between Cortex-A9 SOC's, the difference in performance and benchmarks is either down to the GPU (The Exynos uses the Mali-400, the TI uses the PowerVR SGX 540, the Tegra 2 uses the Geforce ULP, and the A5 uses the PowerVR SGX543MP2), software optimization, clock rate, or certain cases where NEON would provide a benefit.
Performance per clock has the Cortex A9 leading the Snapdragon chip in most cases. This isn't Qualcomm's fault. The Snapdragon was never designed to compete with the A9, but rather, the A8. This is why benchmarks seem to have the T-Mobile SGSII coming in last place between the carrier variants. In real world performance, though, it's relatively a moot point. Most software is still being written for the 1GHz single core crowd. Any dual core device should remain hardware relevant for quite some time.
As far as GPU performance is concerned... the Adreno 220 /should/ be more powerful than the SGX540 the Galaxy Nexus uses, but software optimization can provide a key role here.
I've heard mixed views over the camera in the Galaxy Nexus. From what I've gathered, at its best, the camera can snap clearer, more color accurate shots than the SGSII at its best, but it's easier to take rubbish shots than the SGSII's. The resolution of the CMOS sensor means almost nothing in terms of raw image quality, and it's silly to compare cameras based on it.
The Galaxy Nexus doesn't use Gorilla Glass, but some sort of unknown manufacturer's "reenforced glass", like the Nexus S. I'll hold off until we hear some clear reports on how durable/scratch resistant this glass really is.
The biggest reason to go with the Galaxy Nexus would definitely be software updates, though. Obviously, it's the first phone with Ice Cream Sandwich, which is a massive improvement over Gingerbread in every way. And then, it will get consistent updates, as they are managed by Google instead of the manufacturer/carrier. (Remember how hard it was just to get an official Froyo on the US versions of the original Galaxy S?) Samsung claims that it's trying to clean up its act on that front, but it's not entirely up to them. It's up to the carriers, and they'd rather us sign onto new contracts with newer phones. We'll see.
There's no SD card slot on the Nexus. That's a big thing for most people.
Audio performance is interesting. The Yamaha DAC the Galaxy S II uses has been called a downgrade from the original Wolfson DAC the Galaxy/Nexus S used. I haven't heard reports on the Galaxy Nexus' DAC, so we'll just have to wait and see.
There will definitely be more ROM support for the Galaxy Nexus. Two reasons.
1. It's not split up into four (five?) different models, some of which have different hardware altogether.
2. AOSP Android. Everything running on the Galaxy Nexus has been open sourced from the beginning. (GAPPS excluded). Meaning there won't be any driver issues as per the SGSII.
If both phones were out right now and available for T-Mobile and I'd have to chose, I'd probably pick the Galaxy Nexus just for the ICS and ROM support. That's not to say I'm unhappy with my SGSII, but running a vastly superior version of Android and being able to tinker around with it that much more is worth it (to me) over my own personal preference for the non PenTile SAMOLED display and the Gorilla Glass.
