Samsung Galaxy S II and the iPhone 4S can be considered as bitter rivals as these are some of the most superior phones ever create. The Samsung Galaxy S II also created headlines after 10 million units of the device has been shipped worldwide excluding the United States.
In terms of processor, the Samsung Galaxy S II beats Apple iPhone 4S as it sports a 1.2 GHz as well as a 1.5 GHz (T-Mobile variant) dual-core processor while the iPhone 4S only have a 1GHz dual-core ARM Cortex A9 processor. As for the camera, both phones have an 8MP sensor although the lens of the iPhone 4S supports a wide aperture of f2.4 which would help in low lighting conditions.
Samsung Galaxy S II has a 4.3-inch Super AMOLED touchscreen which is a bigger than the iPhone 4S 3.5-inch Retina display. In terms of operating system on the other hand, a lot of people may find the iOS a lot better and attractive as compared with the messy Android interface which then gives the iPhone 4S an advantage here.
May it be the iPhone 4S or the Samsung Galaxy S II, you would be satisfied with any of these two phones as they are spectacular. As of date, you may not find any phone as good as these.