AT&T, Verizon, and Sprint all offer almost identical versions of the iPhone 4S. The situation is abit more complicated with the Android device -- the Galaxy S II series includes the Galaxy S II and the Galaxy S II Skyrocket from from AT&T, the Epic 4G Touch from Sprint, and T-Mobile's version of the Galaxy S II. These are all basically the same device, with minor differences between the carrier-specific versions, except that the Skyrocket is equipped with 4G LTE.
Build & Design
When it comes to design, I say the iPhone 4S takes the prize. It's small, it's sexy, and everyone knows exactly what kind of phone you have when you pull it out. The Galaxy S II devices aren't exactly ugly, but they aren't exactly head-turners either, suffering from a "me too" design sensibility of all-black devices without any real flair.
Both the Apple and the Samsung handsets are dominated by large displays that cover the front of the device. They also have the same basic button and port layout, though the buttons on the iPhone 4S are slightly larger and easier to operate.
Display and Keyboard
The iPhone 4S is equipped with a 3.5-inch IPS display that has a resolution of 960 x 640 at 326 ppi. It has also been treated with an oleophobic coating designed to repel fingerprints, but you'll still be cleaning off the display pretty frequently.
The various Galaxy S II devices have larger displays than the iPhone 4S, measuring 4.3-inches to 4.5-inches, with a resolution of 800 x 480 (WVGA). They look very good, and being larger than the iPhone they offer more immersive entertainment, but they don't have that same high resolution, razor-sharp clarity.
The Galaxy S II devices do have an edge when it comes to outdoor viewing, because they are equipped with Super AMOLED Plus screens that are capable of amazing brightness. They almost cause your eyes to bleed at the highest settings. The iPhone 4S is also viewable outside, but isn't able to compete on sheer brightness.
When it comes to text entry, you'll be using a virtual keyboard on either of these smartphones, as none of them have a physical keyboard. They're pretty comparable across the board, though the Galaxy S II models have larger screens and therefore slightly larger virtual keyboards.
This is pretty much a dead heat, with the iPhone 4S being slightly smaller than most versions of the Galaxy S II series, but also slightly thicker. The Apple handset is more pocketable since it's smaller overall, measuring 4.5-inches tall, 2.31-inches wide, and 0.37-inches thick, but it is slightly heavier than the Galaxy S II phones since it weighs in at 4.9 ounces.
The Galaxy S II versions vary slightly in their exact measurements, but they are generally larger overall than the iPhone 4S. Sprint's Epic 4G Touch, for example, measures 5.1-inches tall, 2.7-inches wide, and 0.38-inches thick. For those looking for a smaller option, AT&T's Galaxy S II is 4.96-inches tall, 2.60-inches wide, and 0.35-inches thick.
Features and Capabilities
The iPhone 4S is based on the Apple A5 chip, which is a dual-core processor, though the actual processor speed has not been released by Apple. Most reports peg it at 1GHz. It is super fast and responsive, though there's no way to directly compare it to the Galaxy S II phones since the same benchmark apps aren't available for the iOS and Android OS.
The Galaxy S II phones from AT&T and Sprint have dual core 1.2 GHz processors, while the AT&T Skyrocket and the T-Mobile Galaxy S II Titanium have 1.5 GHz dual core processors. They all perform well and also have an edge when it comes to true multitasking, while the iPhone 4S has fast app switching and very limited multitasking (allowing apps to complete only certain tasks in the background when you switch).
When it comes to memory and storage, the iPhone 4S has advantages and disadvantages. Depending on the model you choose, it has more internal memory than any of the Galaxy S II devices, because it's available in sizes ranging from 16GB to 32GB. It does not, however, have any sort of memory card slot to add additional memory.
The Galaxy S II devices have 16GB of internal storage memory and also have a microSD slot that supports memory cards of up to 32GB in capacity. That means a top storage capacity of 48GB, less than the largest (and most expensive) iPhone 4S version, unless you want to deal with switching out the card on a regular basis. That would be quite a hassle, however, since the card slot is generally located in the battery compartment, and sometimes under the battery. For that reason, the iPhone wins when it comes to memory, assuming that the 64GB model is chosen.