Let's first acknowledge that you've asked this question on Android Forums, so you are more likely to get Android-favorable responses. Nothing wrong with that, but you will be better informed if you ask on an Apple forum as well and then draw your own conclusions.
First let's look at the numbers ...
In the first quarter of 2014, Android phones shipped approx. 187 million units, Apple shipped almost 44 million. While the growth numbers are fairly close, remember that it's growth based on units so Android units grew almost 45 million units while Apple grew by 7.5 million. Think of it this way ... Android's growth number is higher than Apple's total units. That's significant.
What I find even more interesting are the
AOSP numbers. At 53 million, they have passed Apple all by themselves. Now, they are based on the Android core code but don't necessarily connect to Google services, so the play store app channel might not be the right delivery model for AOSP devices. But (and this is a big *but*) they most likely will be able to run Android-developed apps whereas they will NEVER run iOS developed apps.
With Android and AOSP, new unit shipments for the first quarter were 5-1/2 times that of Apple. That's quite a market.
Now let's get down to brass tacks ... or should I say "gold tacks"?

It's all about the money. Apple makes a boatload of it. So much so that they've needed to buy more boats just to hold it all. Whether this trickles down to the developer, i couldn't say, but there is a much greater likelihood that an iOS user will pay for an app than an Android user.
On the other hand, a smartphone user is much likelier -- any platform -- to arbitrarily try a "free" app. The highly popular game "Angry Birds" is a good example. It was originally released for the iPhone only as a paid game. IT made Rovio a lot of money, but within a year of releasing the free (ad supported) game for Android, they were seeing more income from the free game. It's really a testament to the success of both the Apple and Google business paradigms.
So what can we conclude? Probably that you can't go wrong either way, provided you have a decent app that enough people want and you have the inclination to market it. If you are just musing with pie-in-the-sky hopes of developing that "killer app" with no real idea, I'd suggest bagging the development courses in any language and take a few marketing courses first.
