Kenneth is right. According to the most recent Nielsen data,
Android has 43% of the market, Apple's iOS has 28%. Android has an
even greater advantage among new smartphone users, and sales of
android phones have increased 5-fold over the last year. Revenue
remains higher for iOS apps, but this data doesn't include
advertising dollars for free apps, which dominate the android
market. If you don't rethink your business plan, you will miss
I do follow these things, but I think the Android numbers are vastly inflated by people who just want a cheap phone, and have no intention of downloading apps (except maybe Angry Birds). Android is basically picking its new users up at the expense of 'cheap' phone manufactures like Nokia, not Apple.
The figures on revenue are shockingly in favor of iOS. iPhone buyers seem to be people who have money to spend -- they choose the iPhone after all -- and are willing to buy apps, because Apple has done a good job on its App Store. As someone who developed for the Mac for years, when it had a 2-5% market share, I can assure you that Apple's 2-5% is a good 2-5% of the market to have; willing to spend money on apps.
Although it would be nice to have an Android version, other developers who have done this tell me sales there are generally much lower. I do not want to go down the advertising line.
I don't have any experience with Android, and the time I would need to spend learning it could better be spent making the Apple products better. I'm sure there are already lots of good Android flashcard apps. If the market is booming there, they must be there too.