I prefer Android. I may be biased, but I don't think by that much. I own an Android phone and I do some Android development. I also own a Mac and an iPod touch, and would get into development on iOS given the time (seriously, "install XCode and get to Hello, World" is actually on my todo for this weekend). I really like all of these devices (my MBP the most though). But I prefer the Android OS for two primary reasons: the back button and the menu button.
When I'm lost in some interface I can usually find my way to what I wanted or find my way out using those buttons. With my iPod Touch, I end up mashing the home button and starting over, which sometimes just doesn't even do what I wanted.
You might say that I am just used to Android, and that's probably true, but I have had the two devices for almost the same amount of time, and use them both daily. I just prefer Android. Yes, iOS is way smoother, the UI is prettier, and the hardware is even nicer in some cases. But when I get lost or struggle with the interface, somehow it really sticks out on iOS. (I should mention that I get frustrated with Android plenty too)
It's the kind of attitude in your last sentence, suggesting that you cannot fathom why anyone would choose the objectively inferior Android OS, that people get annoyed at in these horrible discussions. It boils down to taste, and to suggest that some people's preferences are not true is unfair. (I'm probably doing you a disservice, I'm sure you're not a raving Apple fan. But you know what I mean, I hope)
(I also wrote a long description of an example of getting lost on iOS but it was really long winded so I trashed it. I can explain it if anyone is actually interested though.)