If you want specific numbers, just email me.
Now, there is a whole lot more competition in the App Store and Android is growing a lot faster. So I suspect the real answer is Android, but that is just conjecture and doesn't take into account the iPad.
I wish I could remember where that study was.
Edit: Magneus linked to what I was thinking of
The typical Windows user won't use more than what came with their PC on the day they bought it. Ever wonder why Dells, HPs, and Gateways come loaded with a lot of crap software? It's because ISVs know that your typical user won't spend more than he has to. If it comes with the PC and it's good enough than they'll use that. But that user will rarely head back to the store to search for an alternative.
When software is well written it's perfectly fine to show the developers some respect by paying for it. That's why the Mac shareware community is in much better shape than the PC side.
Whether it's shelf space or disk space ISVs pay for it.
http://tech.fortune.cnn.com/2010/02/25/6-ways-iphone-and-and...
iPhone users have a track record of spending money - the iTunes App store is a huge success. I have seen more stories of iPhone developers making scads of money than I have for Android. Having said that, that is a catch-22. The successes represent around 1% of all of the apps in the store.
The one emerging problem with the iTunes App store, however, is discoverability. It is damned hard to get discovered within the iTunes App store. Clearly to be successful, you have to get a lot of good buzz outside of the App store.
The market demographics are very different - Android : 3/4 male, purchase apps 1/2 as often as iPhone users. iPhone users, around 50/50 male/female, affluent (as Android comes with cheaper devices the owners of those devices probably aren't using the platform the same way as iPhone users do).
Obsolete platform hardware/software - this is huge. Most existing Android phones won't ever get an upgrade to 2.x Android OS. Therefore to reach the market, you need to make it backwards compatible with the 1.x, which is a PITA.
Apple has only just announced that their next OS won't work on original iPhone (2G). This is a massive difference. You have been able to build for the latest OS for all iPhone devices for 3 years. Contrast this to Android - some devices released in the last 6-12 months won't run software built against latest Android OS.
Flash - this is big and I believe it will push prices lower in the Android app store. There has already been a "race to the bottom" in some categories (e.g. games, think 99c variety) in the iPhone App store. iPhone users will pay for games that you can play in a browser on a late-model Android phone for free. Therefore there's less incentive to pay for these games on Android devices.
Discoverability will always be a problem for any platform.
My answer is that it depends. If Apple don't object to your app or business model, then the iPhone is probably the better bet, for now, and until and unless the number of Android handsets greatly outnumber iPhones (i.e. even with a lower customer conversion rate, the absolute numbers then tip in favour of Android).
Factors that limit the money available to Android app developers include a) lots of free apps to choose from; paid apps need to offer something special - even many trivial apps for iPhone seem to be paid b) handset manufacturers' lack of co-operation with Google in getting their firmware releases approved by Google for copy-protected Market apps makes it more likely that potential customers will just download illegal copies of your app because it's actually easier than buying it. Fail.
Any hooks? They seem to ask for a signup to create an app.
Someone had to say this :P
It sounds like your after "code a weekend and get paid millions". Which is a statistically non-existent market on either of the platforms.
What gave you that impression? Or has the submission been edited?
All three platforms have pretty big market shares, focusing on one exclusively seems like bad business.
But my comment was that if he really is after making something valuable, he can choose either platform to make it for first and then make it for the other. It really won't make any difference since he will most likely not just be able to make it for one platform then wait for the money to roll in.
Or in other words, making it for one doesn't have to exclude the other; and after having created it for one, he will be able to get a better feel for the market and work on the other while working on the marketing.
I don't see how choosing just one of the platforms will make you the most money unless you're counting on a lucky break and get the lottery sales figures. Hence my judgment that he's really asking what platform he's most likely to get lucky in.
Of course everything I say has limitations in what you want to produce. And sort of contradictory to my point if the project is very big.