There really isn't a non-diplomatic response that works here. If you just want answers saying one or the other, there's the poll in the comments, but that's just personal preference to some extent - and I'm sure "personal preference" counts as diplomatic too.
The best I can do is that there's more Android devices, but I'm not sure which platform downloads more apps, particularly paid. Last I heard most of the money came from iOS.
If costs are an issue, then Android is likely to be cheaper to get started with, but that depends what you already have. I believe you need an OS X device to program for iOS, if you don't have one and costs matter, that's a big one. If you want a suite of testing devices, Nexus 4, 7 and 10 are cheaper than iPhone 5, iPad Mini and iPad in general. You probably have something from there already though, so that depends. iOS has a $99 developer fee, if costs matter that's another.
So, I won't give an answer that it depends on the kind of app you're developing, but I will say it depends on lots of factors you haven't given us. It might help if you detailed what you have gathered from your own studies on the platforms, and what specifically you wanted from this question.
