You can see on the numbers that the thing android does more then anything else is help expanding the smartphone market (90%+ growth!), selling to people who never had a smartphone before, and taking over symbian, bb and windows shares.
It doesn't seem to be that many that convert from iOS just yet.
If we're talking individuals, it's hard to say - some people have switched from IOS, just as they've switched from Android.
Overall, IOS has been growing - but then, so has Android. So we could just as well say that not many are converting from Android either. (They might be more likely to switch between Android manufacturers, but then that's the great thing about Android, you have that freedom of choice.)
I don't think anyone who actually know anything about the smartphone market have ever thought that iPhone is the largest.
Well indeed, "actually know anything" is the problem though

Though, on the other hand, most people don't even know what a smartphone is, and don't care.
It has been the biggest "large fancy touch-phone" for some time though, which I think is what the customer classifies it as.
But it isn't! I'm confused how you would even think this would change anything, since all Android devices ever have always fitted this definition.
It's true that "smartphone" is terribly ill-defined, but the definition used today and the past few years by the media is one that only looks "large fancy touch-phone". If we were going to get rigorous about smartphone definitions, I'd say it's rather odd that the original iphone is included (which couldn't even run apps), but many "feature" phones aren't (even though they've done apps and Internet for years, as well as yes, touch screens). But still, even with the restricted definition which just looks at a selected set of platforms, Android is way ahead.
Cool that you can get that many downloads on symbian still!
Just out of curiosity, are you selling it, or is it free/adware? Is it a game or an app?
Sadly for free

Interestingly this picture is reproduced over the 3 cross-platform apps I have, one is a utility (1.4 million downloads on Symbian since Jan 2012), the other two are games (one is around 20,000 downloads last time I looked on Symbian since around Apr 2012; the other is a lot less popular, just a simple breakout clone).
I'm not saying anything about if you should or not, just that most people tend to do.
Symbian never tried to market their name to end users though, so its not that strange that many forget about it.
Fair enough, I misunderstood

It's true that "Symbian" has never been a marketed name that Nokia used in their branding.
I do very much not like it though, something I developed over 5 years of working with them. Not just writing apps, but writing code for the actual OS.
That's one horribly over-engineered and very inefficient system, no matter how much their architects tried to upsell it
If one need an example of OOP running amok, Symbian is a good case.
It was obvious to me that they would die, about 4 years ago, a death by legacy, by not beeing able to cope with change
But they did change! Note that things got massively better in 2010 when Qt became the official SDK. It's all I ever used, and I love it (in fact I'm using the Qt port for Android too). I did hear bad things about the older Symbian C++ system.
I can think of plenty of reasons why they might have switched - it's a lot easier to use someone else's OS than maintain it yourself. But despite it's age, it was still doing well. Of course it's true that sometimes things have to be replaced - just as Apple had to do with classic Mac OS, or MS did with DOS, Windows 9x, and more recently with Windows CE. But there's also the question of branding - Apple still call the new different OS X "Mac OS", and Windows Phone in fact switched from Windows CE to NT for the kernel from 7 to 8. So legacy problems don't mean a platform has to die, though I can see Nokia deciding it better to switch to something else, like Apple did when they used Next, but there was little point keeping the Symbian name when they'd never marketed it.
It's not clear to me either who comes out on top, but it should be a pretty tight race if you only look at high end devices over the years.
We're talking about devices that is released as the top end of their product line, so also S2 and iphone 4 is counted.
Since writing that, I've now seen this:
http://www.theregist...ing_smartphone/The S3 alone outsold iphone 4S in Q3 2012. Of course, on top of Apple's totals, we'd have to add the iphone 4 and 5. But to Android totals, we're adding the Note, Note 2, S2, as well as the flagship phones from all the other Android manufacturers like HTC. So it's not proof, but I'd be very surprised. (Plus, does say an old iphone 3GS that's still being sold today really count as a high end device? It seems unfair to count that, but exclude mid-range phones today that completely outdo it.)
For us, as application developers, what really matters is one thing. "cost to develop" compared to how many are willing to give us money for our work.
The really big problem for android is that cost to develop skyrockets when there is so much fragmentation, and the will to pay is still very much lower then on the app store.
I'm certainly not saying there are no reasons to developer for iphone. Everyone has their own reasons - and the reason you give isn't the only consideration either.
The OP asked which was more popular, and I'm just clearing it up that it's Android, by miles.
There are indeed many considerations, and the fact that it isn't simply about popularity is also why one shouldn't just restrict consideration to Android and IOS. Similarly on desktops - Windows is clearly by far the most popular platform, but that doesn't mean there isn't reason for some people to write for OS X or Linux (or a combination with cross-platform software). It's true that the stereotype seems you're more likely to find mugs with iphones who are more likely to splash out loads of money for software even if it's simple to develop, though as a user, I much prefer the greater range of free software on Android.
For my perspective: Having to buy Apple PCs would be extra cost, as would the yearly Apple fees. It's also extra hassle - if I'm travelling, how do I develop if I only have my Windows laptop? Do I have to lug two laptops around with me? Even at home, it's a pain to have to switch to another computer just to write some code.
Whilst supporting more models is extra cost/time, I wouldn't say it skyrockets - if this was the case, no one would ever develop for Windows. Also consider that "fragment" implies small, when really these are huge great chunks - as shown above, even if you restricted yourself to Samsung Android phones, or even just the S3, it's beating Apple.
It's no longer the case that there's one single model either for IOS - you've got 3 earlier iphones, the 4 and 4S at a different resolution, then iphone 5 with a different size and resolution, then 4 ipads with varying resolutions, and now an ipad mini with a new resolution. It's getting to the stage where supporting those devices is no harder than testing the varying range of the Android resolutions anyway. I don't think you need to test every single Android device, just as no one would test every single configuration of Windows PC there is out there.
Plus, even though it may be an advantage, I still much prefer the freedom of choice of what models there are to buy (and develop for).
I develop for Symbian still as the tools are excellent and there's no hassle, and Qt allows me to develop for Android (and Windows and Linux desktop) too. Nokia have a great feature on their website to allow remote testing of any Nokia phone - I do wish something like this existed for Android. I also dislike as a matter of principle Apple's model of controlling the software so that you can't release without Apple's permission (same reason I don't want to develop for WP or Windows RT). I'm not saying everything's perfect on Android (the emulator control options aren't very user friendly, for example), but each platform has its pros and cons (Nokia and WP seem to have good tools too), and if we're looking at more than just installed userbase, just as there's more than just Android, there's also more than just Android and iphone.