It's early days yet in the iPhone 4S versus Samsung Galaxy Nexus comparison wars, especially since there aren't many comprehensive reviews of the latest Android phone out yet. But comparisons based on at least one cross-platform benchmark indicate that the Nexus is the far speedier phone when it comes to web browsing.
Then, on Google's page for developers describing the new Nexus phone and Android 4.0, there's a graph way down the right side of the page giving scores on the same test for the old Nexus S and new Galaxy Nexus. The Galaxy scores 1,983 milliseconds on the SunSpider test, way ahead of the iPhone 4S, and 1387 on the V8 test, almost four times better than the iPhone.
It's also interesting to note that this was decidedly not the case if you compare previous models of each phone. The iPhone 4 scores 3,590.7 milliseconds on SunSpider, mashing the old Nexus S score of 6,223. And the iPhone 4 scores 257 on the V8 test vesus 211 for the Nexus S.