It's more on par with the 4S. 1.2ghz dual core (1.5 on T-Mobile), 1GB Ram, and TouchWiz is even very iOS looking. I think it has a nicer screen, very thin. AT&T's version has the smaller screen, 4.3" instead of 4.5. The AT&T one is more like the one that the rest of the world is getting, and the S2 is supposedly NFC ready, but it's not currently enabled. The S2 is an HSPA+ phone, capable of 21mbps, faster than the 4S.
With the 4S only being 1Ghz and no NFC, I think it's a phone that fits nicely into other phones available today, where the S2 a little bit more of a "tomorrow" phone, and the Nexus will be more of a next week's phone. If you want to get an Android, it would be silly not to get the Nexus IMO.