I think the majority of people are shopping by carrier and going for style/hardware. Not that IOS isn't just as good in its own way, but I think a lot of the IPhone buyers are attracted to the hardware and the brand.
If you look year-over-year, IPhone had impressive growth (as did all), but it's market share remained flat @25%.
People always want to say "what if IPhone had more than one handset". Well, let's turn it around and say "what if the IPhone hardware sold running Android?" An IPhone running Android or IOS on another phone would cannibalize a lot of the existing IPhone sales.
IMO, the brand is powerful. But there's a lot of competition, and side-by-side all else equal I think you'd see upwards of 10 phones (maybe more) taking the lionshare of the market. Within that segment, IPhone branding is probably worth a 2 to 1 share.
One of the things being ignored in this debate is a lot of people who want an IPhone are already tolerating AT&T's service. If you don't want an IPhone or something else is more appealing you've probably left AT&T by now.
The other big thing being ignored is the branding/marketing does have a huge impact, but IOS on another phone IS NOT an IPhone, not to mention the fact that there is only one IPhone and only one cell running IOS is critical to the marketing/branding strategy.
And the main reason Android is popular with so many manufacturers is that it costs them nothing. They make more money while still being able to price competitively. I don't think there's a huge market of potential buyers out there (that don't already own an IPhone) willing to pay an extra $100 for a cell running IOS vs. the same hardware running Android. It's very difficult to make money competing on volume with something that is free, and this is why I don't think you'll ever see IOS licensed to more than a few, if at all.