Nice thread! Actually it should be Android vs iOS vs Windows.. but yes, since Apple is only making 1 type of phone at the moment we can very well say iOS and iPhone are the same thing...
So..
I will vote for iPhone, but don't get me wrong, I'm not a fan-boy, I have tried the Galaxy S2 before (friend of mine has it so I often see it and use it) with an Android operating system... and there are some things that are way better on my iPhone:

1- User friendliness: can't explain it, it's all so well organized, no confusion, if I want to do something I do it in a matter of seconds with no lagging, no waiting, no buffering, no searching.

2- Stability: It's 3 years now ( iphone 4 and iphone 4s) and NOT ONCE my iPhone crashed. Need I say more? I'm sure I'm not the only case.

3- App store: You find everything there and what you find there WORKS 100% on your iPhone. Perfect fit on screen, perfect usability, perfect compatibility. Infinite number of apps available and quality and security is always at it's best.
You don't need to check apps, Apple does that for you. You just download and install them.

4- Materials: I had time to unmount a friends iPhone and my friends, not only the exterior which is gorilla glass, but also the interior is just an engineering masterpiece, from the antenna located around the phone, to the location of all silicon pieces. You can see it's high quality.

5- Speed: Last but not least, in terms of speed I never suffer any kind of lagging, especially swiping through pages on my homescreen on the iPhone, I don't know why but on my friends Galaxy S2 I have the impression that the swiping is taking that 1 microsecond to much to get started that really feels annoying coming from an iPhone experience. Same goes for opening folders. I seem to see a small lag if compared to my iPhone. Sorry if I can't explain better this one. It's just a "feeling".

That's all folks.