While I'd marginally hand Android the edge in getting its platform up and running the OS is set for its biggest fall in this category. In itself this is a remarkable statement as the Android UI and navigation system is streets ahead of the rest of the market (I don't care how many generations of third party manufacturer skins you slap on Windows Mobile). The problem is the iPhone resides in a different league even to that.
Surprisingly, on the surface then there isn't much to pick between either. I'll admit a slight preference to the style and layout of mobile OS X but the fact Android is free to be customised in any way a user sees fit means the latter is likely to win out in the long run. As for layout, both employ grid based desktops where icons can be freely reorganised or even dragged off onto secondary desktops enabling programs to be grouped in whatever way a user sees fit.
In actual fact, Android has a little nugget of gold here as rarely used programs can be 'binned' and consigned to a slide away panel which acts much like a secret desk drawer - keeping largely out of sight unless requested. It's an option mobile OS X could do with since in my case I can't remember the last time I used 'Stocks' (good thing, methinks) - and I'm sure every iPhone owner has their own redundant functionality they'd like to file away.
On the other hand, where the iPhone streaks ahead is navigation and it is because of that old chestnut Multi-touch. For those of you who have been living on another planet these last two years, multi-touch essentially amounts to commands using multiple fingers so , for example, whereas Android restricts users to hitting zoom in and zoom out buttons mobile OS X can zoom out wherever two fingers are touched to the screen and 'pinched' together while reversing the motion zooms in. This may seem simple, but it makes navigation of web pages, Google Maps and pictures a joy to behold.
Underestimate the value of multi-touch at your peril. There's a reason it won TrustedReviews' Best Technology of 2007 award.
That said, mobile OS X is also a revelation when it comes to a much simpler and underappreciated command: double tap. It may not be as flashy as multi-touch, but double tap is just as useful functioning as a kind of 'smart zoom' which locks onto paragraphs of text or images in the browser, zooms a few stages at a time into Google Maps and toggles zooming in/zooming out of photos and even videos.
If Android is to close this gap, a multi-touch update of its own has to be right at the top of Google's priority list.