Android for me.
First because of awesome customization. Just look at
this video where you can see my animated Smash background and the behaviour of my bottom bar. Yes, that bottom bar is completely counter intuitive, but it doesn't matter at all since
I was the one to design it so I know exactly how it works, I don't have to guess. And it's now ultra convenient. Also, that screenshot is old. I now have a navbar at the bottom of my screen so that I still have access to navigation buttons even when I hold my phone upside down, which is convenient to do when it is charging on your desk and the cable is less annoying at the top.
Second because it is more open. If I was to develop for mobile devices, it would be tons easier on Android. Just do my things and test them, then if I want to publish on the Play Store I only have to pay for a license once. Now on iOS from what I understood, well I can't test my own app without jailbreaking or paying a license first, and that license isn't even a one time fee, it has to be paid several times.
Third because it's either cheaper for what you get or better for what you pay for. I mean, if you go abroad and only need a phone with dual-sim but not necessarily powerful, why would you pay a lot of money ? Just get the cheapest dual sim Android you find. On the contrary, if you want to play games all day, get the most powerful Android you can find on the market. With iOS, you don't have the choice, it's either too powerful and too expensive for what you do with it or too limited, but it's not often "perfect" (except that most people are in the first case, so if they have too much resources, they don't complain, they just wasted money but they don't know it).
Fourth... because I just don't like Apple's philosophy. I bought a Macbook this year to do Java programming (because Java is the most annoying thing to install) and it was the first time I ever saw a computer ask me to get an account just to install an OS. Windows doesn't ask that. Linux doesn't ask that. OS X does. And guess what. I made an account, but it didn't let me use it. It first wanted me to get something on iTunes to activate my account. But how am I supposed to get something on iTunes if you don't let me use my Macbook ? Yeah, they seem to think less about convenience than about getting people to spend first and spend more after getting in the ring, you know ?