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Picking a new iPhone: there's only one decision that matters
To understand how important (or not) OIS is in a phone, it's important to understand how it actually works and what it actually does. OIS has been around in DSLR lenses and point-and-shoot cameras for over a decade, and other smartphone manufacturers have been putting it in their devices for years before Apple jumped on board. It is designed to counteract hand-shake and camera movement, by literally moving elements of the camera's lens in the opposite directions of the photographer's movements. This allows for taking sharp pictures with longer shutter speeds than would otherwise be possible. Longer shutter speeds let more light into the camera's sensor, allowing for brighter pictures in low light environments. It's long been billed as the savior for low light photography and it lets you use a really big zoom lens on your DSLR or mirrorless camera without a tripod. OIS is most effective when you have a long focal length, because that magnifies any uncontrollable movements you might have.
But, and here's the key misunderstanding with OIS, it is only able to correct for movement on the camera's side. If you have a longer shutter speed, unless your subject is inanimate and perfectly still, it's more likely that your subject will move in the time that the camera is taking the picture, causing motion blur that the OIS system can do nothing to correct for. You've seen this before, often times when you try to take a picture of a frisky pet or active toddler indoors, they move during the shot, causing a blurry limb or loss of sharpness in key areas such as the face.
Having OIS in a camera does not resolve this problem, and in fact, a lot of smartphone manufacturers have exacerbated the issue by relying too heavily on the feature and holding the shutter open for longer than would otherwise be necessary. (I've personally seen this with every smartphone I've reviewed that has OIS, including models from HTC, LG, and Nokia.) And since smartphone's have wide lenses, OIS is less effective than it would be on your DSLR with a 300mm zoom lens.
The fact is, most of the pictures people take with smartphones are of other people, which are living, breathing, and yes, moving subjects. The most important factor in getting sharp pictures of people is shutter speed — it needs to be fast enough to freeze the subject.
That isn't to say the OIS feature has no purpose — it's really great for getting sharp pictures of buildings at night or those cool nighttime cityscapes. They aren't moving and will surely sit still long enough for the camera to use a longer shutter speed. OIS can also be effective for stabilizing video. It can also be argued that OIS is more effective in the iPhone 6 Plus (and possibly more necessary) than it would be in the iPhone 6 because its larger size can be more difficult to hold steady.
By all accounts, it appears that everything else about the cameras in the iPhone 6 and 6 Plus are the same. According to Apple, they have the same resolution, same sensors, same focus system, same lens, same flash, and same new camera processing features. They also have the same video recording features, including slow-motion, 1080p recording, and something Apple calls "Cinematic Stabilization."
But in the vast majority of still photography situations, these cameras are likely to produce the exact same results.
