In this video we put the Samsung Galaxy S 2 up against the iPhone 4! In terms of price, the iPhone 4 is currently a much better value: it's priced at $200 and $300 for 16GB and 32GB versions, respectively. The Galaxy S 2 costs over $700 unlocked right now, but is likely to come down in price once it's available on a carrier.
In terms of build quality, the iPhone 4 is superior because it makes use of metal and glass, while the Galaxy S 2 is plastic and glass. Both have a high quality feel in-hand.
The screen of the Galaxy S 2 provides better contrast, but not as good of a viewing angle as the iPhone 4. We also found colors to be a bit more natural on the iPhone 4.
In terms of web browsing speed, it's no contest: the dual 1.2GHz Exynos CPU and the 1GB of RAM makes the Galaxy S 2 very quick when browsing the web.