Given that (at least for now) you're in it for fun, and that you already have Mac hardware, seems to me iOS is a good place to start. With the combination of the iPhone simulator and your iPod Touch you have what you'll need to dive in.
One thing other posters haven't brought up is the abysmal condition of the Android emulator. Unless you have an Android device on hand, you're in for a world of pain trying to develop apps using the emulator. It's snail slow, with a long boot-up time & very poor graphics performance. (It's something the Android dev tools team at Google is working hard on fixing, but it's a ways out, from my understanding.)
Finally, if you're the type who likes a structured approach to learning there's a great course that Stanford University offers covering introductory iOS development (http://goo.gl/SouE) to help get you started.
One thing other posters haven't brought up is the abysmal condition of the Android emulator. Unless you have an Android device on hand, you're in for a world of pain trying to develop apps using the emulator. It's snail slow, with a long boot-up time & very poor graphics performance. (It's something the Android dev tools team at Google is working hard on fixing, but it's a ways out, from my understanding.)
Finally, if you're the type who likes a structured approach to learning there's a great course that Stanford University offers covering introductory iOS development (http://goo.gl/SouE) to help get you started.
Good luck to you, my friend!