[Vibrant Q&A, Help & Troubleshooting] eksasol:
I had the Note (Exynos 4 1.4ghz dual) and S3 (Exynos 4 1.4ghz quad). Overall, I think the Nexus (OMAP 4 1.2Ghz) should be comparable in performance with the SII and Note in real life practical usage. All of these CPUs are just modified variation of the ARM Cortex-A9 so they are really of the same generation.

The SII has a resolution of 800x480, so maybe the Nexus had to work a little harder with the 720p screen. (There is zero reason to get an SII right now when you can get a Nexus with 720p screen, this is a huge improvement in usage convenient.) Overall, a stock Nexus has a faster interface experience than a stock Galaxy S3 because it is running stock Jelly Bean compared to the bloat Touchwiz S3. The software does play a big role in performance, the proof of this is the iPhone 5, its CPU spec is a lackluster, but they tune their OS to be very smooth. The firmware image of the S3 is nearly 1GB, the image of the Nexus is ~180mb. The S3 do manage to be faster once you install custom rom though.

On the GPU side, the S3 have a quadcore Mali-400 MP4. The Note's GPU is similar, but is single core. The S3 will perform much better with heavy 3D gaming compared to the dated PowerVR SGX540 that both the Nexus and Vibrant are using. For video playback, the OMAP 4 have a dedicated DSP to playback HD videos smoothly, so no problem there.

Personally, I think there are more factors to make a phone good than just performance. I didn't like the S3 as much as I thought because of the poor built quality and not getting the latest developments that one would get with a Nexus, its still a very good phone though.

About considering to get a Nexus, don't listen to your friends on this, they might be swayed by the newer phones with flashy specs, but in the end of the day, Nexus devices will always get the most development and latest updates. In a year or two, many devices released now will become unsupported and manufacturers will no longer update them, yes you can still rely on custom roms but they can come slower and less stable. But with a Nexus device, Google will continue to release updates for it and you will get the latest version of Android before other phone owners. Also there will be lots of developers working on it and the roms for Nexus devices tend to be high quality. Another advantage of the Nexus is its pentaband, so it support both ATT and T-Mobile 3G. I regretted selling my Nexus to buy an S3.

There is one major major downside of the Nexus is that it lacks microSD card, this also means it use "MTP" transfer mode instead of USB mass storage. Both of these facts annoy me a lot, but it might not be an issue for others (PS. The S3 also only use MTP mode despite having microSD slot). I rather use my backup Vibrant right now than to rebuy a Nexus because I can store my music collection and its Wolfson DAC have great sound quality. I'm waiting for the next Nexus to be announced before buying anything, I suggest you do the same because rumor is its coming by the end of this year. I'm also tempted by the OPPO Find 5's 1080p screen and 12MP camera, but I tend to only buy phones that are supported by CyanogenMod.