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Originally Posted by ghak99
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It's not just the extra f/. The f/1.4 has the ultra-sonic motor, means faster focusing. It's also got 8 diaphragm blades instead of 5. This is what that means:
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The diaphragm usually has two to eight blades, depending on price and quality of the device in which it is used. Straight blades result in polygon shape of the diaphragm opening, while curved blades improve the roundness of the iris opening. In a photograph, the number of blades that the iris diaphragm has can be guessed by counting the number of spikes converging from a light source or bright reflection. For an odd number of blades, there are twice as many spikes as there are blades.
In case of an even number of blades, the two spikes per blade will overlap each other, so the number of spikes visible will be the number of blades in the diaphragm used. This is most apparent in pictures taken in the dark with small bright spots, for example night cityscapes. Some cameras, such as the Olympus XA or lenses such as the MC Zenitar-ME1, however, use a two-bladed diaphragm with right-angle blades creating a square aperture.
Similarly, out-of-focus points of light (circles of confusion) appear as polygons with the same number of sides as the aperture has blades. If the blurred light is circular, then it can be inferred that the aperture is either round or the image was shot "wide-open" (with the blades recessed into the sides of the lens, allowing the interior edge of the lens barrel to effectively become the iris).
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I've owned both of these lenses, with my current being the f/1.4. It's notably better built than the cheaper one, but the money may not be worth it to you.
The biggest downside to the cheaper lens is it has a plastic lens mount. This isn't a big deal if you're going to put the prime on your camera and leave it there, but I'd consider the more expensive version if you're going to swap lenses with any regularity. Or, check eBay for a Mark 1 version of that cheaper lens (it has a metal lens mount).