The Galaxy S III has an even bigger screen (4.8 inches, providing 720 by 1,280 larger pixels) -- one that dominates the curved and coolly embellished plastic bezel. Its design is also simple, but unlike the iPhone 5, it's very bold. An iPhone 5 has the vacant feel of a runway model's facial expression, whereas the S III is the life of the party. The S III's screen is also brighter, its colors more vibrant. The iPhone 5's screen has a yellowish colorcast that makes whites look dingy when its brightness is set to medium or low. (The iPhone 4's colorcast is bluish, providing whiter whites, but making browns and oranges duller.)
I prefer the S III's screen. If you have middle-aged or older eyes, the bigger text of the S III (due to the fact its screen is bigger, as are its pixels) is easier to read than the sharper but smaller text of the iPhone. If you're in your 20s or 30s, the readability difference probably won't be apparent. The iPhone 5's Retina display does present text more sharply, which is helpful when reading the tiny text in so many apps.
The S III's larger size comes with a price: It's hard to use one-handed. Not only does iPhone 5 fit better in your hand, but its screen is accessible by your thumb. For the S III, only the Hulk's hand is big enough for the thumb to reach the full screen.
As you'd expect, the iPhone 5 sports a faster processor -- Apple's own A6 -- compared to the previous iPhone, as well as faster graphics processing. The speed advantage is hard to notice in practice, but various benchmarks show there is a difference. The S III also has a beefy processor and graphics subsystem, but it's hard to compare to the iPhone 5, given that the applications are different. The bottom line: Both are fast enough.
The iPhone 5 camera's optics have also been improved, resulting in finer detail, especially in low-light conditions. But the new lens has apparently caused lens flares under some circumstances. The S III's camera is unremarkable. Both are fine for most snapshots, but if you want your smartphone to replace your digital camera, you'll prefer the iPhone 5.
The iPhone 5, like its predecessors, has no removable storage. You choose 16GB, 32GB, or 64GB of internal storage when you buy it (with no-contract prices of $649, $749, and $849, respectively), and you must live with that choice. The Galaxy S III, by contrast, comes with either 16GB ($630) or 32GB ($680) of internal storage, but it can take an SD card of up to 64GB capacity as well.
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