Definitely iOS. Feels more like a properly designed smartphone OS rather than trying to be a desktop OS shoved onto a mobile device, which is the feeling I always get when using Android.
This is actually why I prefer Windows Phone to both iOS and Android in many ways. iOS and Android aren't that differen't. iOS' blinding-white menu system
still looks like Mac OS' shrunk down, it's home screen is just as conventional a icon-grid as Android's (but no widgets), and it had to borrow features like a notification shade/center (which they didn't do very well, in my opinion) from Android. Android just looks more Linux-ish and iOS looks more Mac OS-like. Not surprisingly, given they are both really rooted in those OS'.
Meanwhile, Windows Phone with it's tile interface really does feel like a mobile-first ecosystem. Ever try using your phone on a dash mount? Well it's twice as easy to launch apps and navigate the operating system with Windows Phone than it is with iOS and it's tiny touch targets and wasted space around icons. iOS' icons are all static and don't give you any information, unlike live tiles which are heavily customizable. Microsoft's original mobile OS (Windows Mobile) pretty much was a scaled down Windows interface and it was not ideal (nor did it have multitouch), but they designed Windows Phone from the ground up to be mobile-friendly and easy to use.
Android isn't much different from iOS in usability out of the box, but it is infinitely more customizable. There are ROMs, apps, launchers and themes that can change every aspect of how it looks and feels. You can change icon size, remove icon labels, add useful widgets, heavily modify the lockscreen, change notification area icons, and a host of other things.
Of course, that
can take some work, but you have the freedom to do things you can't on either iOS or Windows Phone which are more locked down.
iOS' greatest strength isn't it's UX or multitouch gesturing language. It's that it has a rich ecosystem of apps and that you don't have to live in it's home system. It also runs on very competitive hardware, right down to having core advantages over Android like audio latency which makes it better for music apps than Android.