If you're not sure, why not start with something like Appcelerator Titanium? It's cross-platform (iOS, Android, preliminary support for Blackberry and even desktop apps) to a very large degree (some UI tweaks may be needed to make it look right) and you can write your code in Javascript -- which is a very useful thing to know even if you don't stick with mobile apps. There are other similar packages (PhoneGap, etc.) but from what I've seen Titanium is much more performant.
To be honest, you'd be better off with making Mac apps. The competition is much lower in that market and it's a lot easier to get visibility for your app on the Mac App Store.
The dynamics of the iOS and Android store have changed a lot in the last 2 years and now the majority of high ranking apps are from companies that have built up a huge cross promotion network.
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.
What's your background? C/C++ then go with iPhone. Java then go with Android.
What's your goal? If you want the path of least resistance to meaningful revenue go iPhone. If you want to integrate deeply with the OS in a way Apple would never allow go Android.
My friend and I are starting as Android developers. Neither of us have previous experience on either platforms. He's got SAS experience and SQL, me I do more web design. So far it's going good, specially for him as he's learning fast. We choose Android as it's only $25 bucks to become a registered developer, plus he has a HP Laptop, no Mac (i have a mac) and he currently doesn't have a job. We do intend to try iOS development if we get Android development working.
Wow yeah, sort of uncanny how that post gives such a detailed answer to your question. The guy says the Android market made about 75% of the Appstore revenue. Fortunately (Or perhaps unfortunately since you would have to develop 2) both markets seem to be worth the effort.
If you're in it for the money (passive side income etc.) I would go for iPhone. There are people who have made considerable income from ads on Android, but as usual this differs between apps and situation.
> If you're in it for the money (passive side income etc.) I would go for iPhone.
I wonder why this is. The culture of those who would buy Apple, would by apps? $100/y dev fee ensures greater quality apps than Androids 1-time $25? iPhone maturity?
In a nutshell? Macs and iPhones and iPod Touches and iPads are expensive. Their owners have more disposable income, which is why they end up with a $650 iPad instead of a $250 netbook. It's also why they buy more apps.
If you're in it for the money, one question is whether or not you have a Mac and iPhone. iOs may gross more revenue, but if you have to spend $1000 just to get the necessary hardware to develop, then that's going to cut into your net revenue deeply.
If $1000 seems like too much to get started, then this is a hobby, not a serious attempt at starting a business.
I've been doing iOS exclusively for the last year but I think at this point I'd steer a beginner towards Android. The learning curve is shallower and the market is less saturated with quality apps. The two platforms are really not so different in many ways so much of what you learn on Android you can transfer later.
I would go for iPhone. Mostly because the tooling is easier, interface builder is cleaner. Android is great too but a litle more involved to get the same result. At least that is my 2 cents.