I respectfully disagree. The CPU in both phones are exactly the same (dual-core A9 @ 1.2ghz). So, that leaves the GPU, and perceived performance differences in the UI due to hardware acceleration. To date, the new Touchwiz was king in terms of UI consistency an fluidity. ICS looks to meet or exceed that, which would be pretty impressive. That leaves the screen and the GPU as the main differentiation, IMO.
In terms of screen, it's pick your poison. SGS2 uses an 800x480 4.3" or 4.5" non-PenTile display (depending on your carrier or international version). The Nexus uses the PenTile display, but 720p and 4.65". So, larger screen capable of 720p, or higher quality but smaller and lower resolution screen?
As for the GPU, it's the PowerVR SGX540 (Nexus) vs. the Mali400MP (SGS2). Now, here's an area that's been blown way out of proportion. The Mali has higher fill-rate, meaning that it scales to higher resolutions better. That's useless on an 800x480 display. The SGX540 has higher geometric performance, which scales better as newer games become more detailed and demanding in that regard. Since games are often set to run at a device's native resolution, the SGS2, due to the lower fill-rate requirement, plays games better today. But, due to the geometric differences, that likely will change in the future.
So, in that regard, it's also a case of "pick your poison." The SGS2 is an amazing phone, no doubt about it, but these two devices are far closer than I think that you implied. If I want a straight up media device, I'd lean towards the Nexus. If I want a gaming device, I'd go with the SGS2 for the simple fact that I often recommend purchasing something based on what it can do now, not what it might do in the future. That lower resolution screen is still more than adequate for games and won't hinder performance to the same degree that a 720p display will.