Windows phone
Why?
It is so much better. I have an iPad, and I've used iPhones. Windows Phone is a far and away superior user interface. And as far as aesthetics, neither iPhone nor Android (which is just an direct imitation of iPhone) are even playing in the same league. And that's important to my graphic designer wife. Honestly, she and I are as amazed as anyone that Microsoft would beat Apple in the graphic design department.
The one place where Android and iPhone have a huge advantage is apps. Windows phone has ~80,000 apps while both Android and iPhone have close to or in excess of 500,000 apps. This is important. There are some very problematic holes in the Windows Phone Marketplace. In particular, there's no Pandora, Audible, Dropbox, or Grocery IQ (though a beautiful official Audible app will be released any day now). Other options exist, like Slacker Radio instead of Pandora and a third party Boxfiles app to interface with Dropbox, but that's not as good as having the real thing. But, honestly,
most of the major apps are there. For a power user there may be some more obscure, but nevertheless must-have, apps that are missing from the Windows Phone library, but for most people all the important apps are there complimenting the core apps.
Which brings me to the core apps, which are phenomenal. Everything from Mail to Messaging to Calendar are exceptionally designed, both in terms of beauty and UI functionality. The People Hub (Windows Phone's version of the contact list) is nothing short of revolutionary (and while I haven't actually seen it, I have the impression that Android ICS may be borrowing the idea). When my wife first got her new phone it automatically populated the People Hub with her Facebook friends and Gmail contacts. Since she has a bunch of Facebook friends that are, like, old high school acquaintances whom she doesn't care about, she told the phone not to show Facebook friends in her contact list. But the phone still pulls data from Facebook for everybody still in her contact list. So when she clicks on any given person, the main page that pops up shows the person's Facebook profile pic (unless she tells it to use a local pic or something from another service), and it shows any email addresses or phone numbers the person has, and gives you quick options to call, SMS, email, or chat with that person. A swipe to the left, though, will show that person's activity on social networks. Another swipe will show photos of that person (albums from the person's Facebook profile, photos they're tagged in--even if you've told your phone not to show Facebook friends in your contact list). And it is all extremely fast, seamless, intuitive and beautiful.
The People Hub is just one element of the UI. The live tiles on the home page are brilliant. The calendar app is excellent, and automatically populates from Google calendars, Facebook events, etc. The music software is excellent.
In short, I think that the core experience of Windows Phone, including all of the core apps (which includes Mobile Office, BTW), is WAY ahead of iOS and Android. The App gap is huge, but in my opinion it's not as big a deal as the numbers make it seem. About 99.999% of iOS and Android apps are utter garbage. Literally . . . if there are 500,000 iOS apps and only .001 of them are worthwhile, that means that there are 500 worthwhile apps available for iOS. That may actually be far too generous. But at 80,000 apps it's not so hard to imagine that a good portion of those 500 worthwhile apps are also available for Windows Phone--the most popular apps, of course, are the most likely apps to be ported over. It's also worth noting, for perspective, that Windows Phone has only been on the market since November 2010, or less than a year and a half ago. I think it took Android a lot longer than a year and a half to get up to 80,000 apps. And Microsoft is throwing dumptrucks full of money at developers right now to encourage faster app development for the platform. Microsoft is an OS company, and they can see as clear as the next person that for more and more of the public, people's phones are becoming their primary computing platform. Microsoft knows that it can't lose this race. And on top of all of that, Windows 8 for the desktop comes out this fall and it shares the Metro UI with Windows Phone. It's a spectacular UI and I think that people getting familiar with it (and possibly falling in love with it) on the desktop will drive sales of the mobile platform as well.
I was close to getting the Lumia 900 for myself, but there are still a couple limitations that held me back. First is that Microsoft currently limits the screen resolution to 800x400 on any Windows Phone device. Also Windows Phone is only compatible with single-core processors. Both of these shortcomings will be addressed this fall by Windows Phone 8. My wife was so sick of her stupid feature phone, that she wanted to upgrade now. She was planning on an iPhone, but I told her that I was going to go with Lumia, but decided to put off upgrading my phone till Windows 8 came out this fall. She was like, "You weren't going to get an iPhone?" So she went to the AT&T store and took checked out the Lumia and fell immediately in love with it. She took a lot of flak at work (design firm--everyone . . . every single one . . . has an iPhone). She just laughs at them and says, "Whatever, brand whore."
And so on, lol. Here's a video of the UI: