A comparison of the latest versions of Apple IOS and Android does not give the same answer as an equivalent comparison done a few years ago.

Google’s Android OS has become the #1 operating system shipped with smart phones and tablets worldwide. This is a merit-based result. With the release of Android 4, the new Holo Light and Dark interface provides users with a clean and sharp UI on their mobile devices.

On UI specifics, the majority of users still say they prefer the appearance of the IOS interface on Facebook.  When viewing friends’ requests on Facebook there is nothing to choose between the two systems. On Twitter and for viewing @ messages there is again nothing to choose between the two systems.

Android works better with Google Translate and also with the sound of a foreign language.  It also does better for those apps which allow you identify the name of a song from its sound while being sung. 

For Geocaching most people distinctly prefer the IOS interface. However, when it comes to selecting a cache you had previously found Android has a 100% success but IOS only 40%.

And so it goes on to and fro. Any thorough and independent test today, by whomsoever it is done, ends up with Android in front by a modest margin. There is a big element of subjectivity in the differences.

Interactiondesign-wise there are important differences, but in terms of UI (your question), the differences are less obvious. Had you asked a few years back, their respective UIs had their distinctions. Much less today. I'd prefer to say that in terms of UI design, no specific design line-in-the-sand can be drawn. Mobile apps are converging into a much more diverse and less guideline driven style, not specifically relating to the platform.

Besides the back button and home button, as an iOS dev vs an android dev you have basically two distinctions when developing on either frameworks. The distinction can be summed up by saying that iOS developers have to be pixel perfect in what they do.

Even with interface builder and layout constraints, you still have to layout each item in the screen manually and make sure it's certain specific "pixels" away from the edges on either side and etc.

Android developers are also able to get the same pixel perfect layouts (so I've been told) but because they were designed originally to support unlimited types of screen ratios, Google opted to make the visual layout of android ux more like html (but with xml).

This results in a much faster screen building on android, but a lot less control and harder to customize views layouts.