Have you found any researched reports on Android vs. iOS market share? That's where I'd start if I were you.

I, personally, would never choose to only focus on iOS. Apple's walled garden seems extremely hostile to me. If you ever got banned for some reason, what would you do?

At least with Android you can distribute .apk files via a website, even if you wouldn't get the visibility being in an app store might give you.

You also might want to consider which platform is easiest for you to develop on. Since you're just a one person show, that might be the most important aspect.