Starting off with android is a straightforward decision because it costs you exactly nothing, you develop in java, on eclipse with the android sdk which you can dowload with on eclipse, and you can post your apps to google play for free.
To develop for apple you need a mac computer, a paid developer account, and will need to figure out objective c, which is no big deal, but not as straightforward as just using the java stuff everyone is already used to. You are also subject to more strict rules for approval of your app to go into the app store. All in all a lot more hoops to jump through if you just want to mess around for a little while before diving in.
That said, the potential for making money off of apps is a lot greater with apple.