There are many factors to consider, for example, when is the largest market, and so on. But ignoring these factors and thinking about technical and monetary issues, a clear answer starts with Android.
Android SDK is completely free. IPhone dev tools need a Mac, so if you are not a Mac user, you need to buy a Mac. If you are not a Mac user, then this is probably a safe bet that you do not yet know the Objective-C that you will need for the iPhone. You don’t even need a phone, there is an emulator that works wonderfully. It is very rare that I did something that works in the emulator, but does not work or works differently on a real phone. Thus, the emulator works great.
Android programming is Java and is very similar to C #.
Android development is much more affordable and easier to develop (for starters, at least) than Objective-C and iPhone.
There are many online resources available, but the book Hello, Android is actually very good. However, it dates from version 1.5 of the SDK, and today we have 2.2, but the basics for getting started are almost the same.
You can go the other way, but in a few hours you can write your first Hello World Android program for free. Even if you decide to start working on a different platform, you are unlikely to be mistaken in making Android the first.
Another thing worth noting is that Android is easier to sell and distribute your applications than iPhone, which makes it better to get started. There is no process to announce applications with an Android site, so you can host your application for free without wondering if they will approve the credentials that will approve your first application for sale or give away.
It should be noted that if you go along the Microsoft route, your experience with C # will be almost completely transferred, and you will be amazed at how compact the SDK is for writing simple Windows applications. (At least that was in 2007, the last time I wrote an application for a Microsoft phone.) But forget that I did it - if you want to become a serious developer of phones for consumers, I recommend that you forget about Microsoft at least least at the moment.