You say you want to (a) learn something new and (b) make some extra income.
As far as (a) goes, your barriers to entry with Android are probably lower. You can develop on Windows, Linux, or Mac; the sdk is free, and there are no charges. Android development is usually done in Java, which is not that different to the c# you already know. So, I'd say get Reto Maier's book and give Android a try. At some point you'll need an Android phone, but you can get some way using the emulator. You won't have to buy a mac or pay for a developer licence.
Once you're familiar with developing for a mobile platform you'll have a better idea of what it takes to build apps that other people will want to use, and maybe even pay for. At that point you can evaluate the platforms from the point-of-view of (b) and decide which one to pursue. If you end-up buying a mac and paying for a development licence then at least you'll be making an informed decision. But get some experience first.
Like you, I'm a c# dev. I've done some Android development for my own amusement, and (for what its worth) my personal opinion is that its a superior platform in comparison to the iphone because it is more open (technologically and commercially). I believe Android will fairly soon either achieve partity with, or even overtake, the iphone.
Try Android, get some mobile experience, then decide.
