iOS
Apple's iOS has a well-integrated ecosystem, a very full apps marketplace, and a fairly intuitive interface, but you're pretty much locked into iTunes for content. If you already have a Mac, an iPad, and/or an iPod, it's probably easiest to go with an iPhone.
iPhones have the advantage of receiving the same OS upgrade at the same time, and the newest OS is usually available on multiple devices. iOS 7, for instance, will work on the iPhone 5S, 5C, 5, 4S, and 4 but not on the iPhone 3GS or earlier.
Android
For its part, Android is the most customizable OS and a wonderland for tinkerers . However, most manufacturers and carriers add a specialized twist, which can lead to slower OS updates, and to an interface that may require a little more ramping-up to do straight out of the box. On the other hand, phone-makers can use that custom layer to add a lot of software features that others just don't have.
True Android devotees should spring for the latest Google Nexus handset, or a Google Play Edition of a flagship phone.
Windows Phone
Then there's Windows Phone. Microsoft's OS has a simple, appealing interface, and Windows Phone 8.1 helps level the playing field with the Cortana voice assistant and a notifications center. Power users still may not find it quite as deep or as flexible as Android, and its app ecosystem isn't as robust. Still, excellent hardware choices give Windows Phone a lift, and it has strong integration with Office and Microsoft's other services.
Do you shop by phone or by carrier?
It used to be that if you were happy with your carrier, or were within an upgrade window, you'll probably pick from your carrier's choices of phones. And if you were off-contract or in between contract cycles, you'd be more likely follow a particularly compelling phone.
Things are a little different now, with T-Mobile joining the off-contract ranks and spurring other US carriers to slacken their vice grips on the 2-year contract structure (and 3 in Canada) with a whole heap of upgrade and payment options.
Then there's the question of the phones themselves. The practice of selling interesting devices exclusively with a single carrier have shriveled, and today, providers large and small stock the most desirable handsets. I still think the people who wind up sticking with their carrier long-term should choose from among those options, but there are other things you have to consider, too, like which carrier covers your area best.
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