Well, this is a very tricky question. I develop for both iOS and Android. Android has a bigger target market. I have built the same app for both android and iOS and gave both away for free. And the number of downloads on android far out ways that of iOS. But iOS apps do sell a lot better. As most android apps are free whereas iOS, most are paid for.
But, if you app is popular, you can make lots of money from advertising. if you look at Angry Birds for example. They charge for the iOS version, and give away the Android version for free, just serving ads. And they make a lot more money from android ads, than charging on iOS. But, the chances of that happening often, is very rare.
Android is a lot easier to code for. The environment is a lot better and stable, even with eclipse being as dodgy as it is. Do not listen to people who tell you the android is fragmented and harder to code for, etc. That is far from the truth. Android is no more fragmented to that of iOS. The best is to target lowest API and upgrade if you need specific features. Even then, you can use reflection to get it to work.
Dealing with the different screen sizes is not hard at all, and does not take a lot of time as some people seem to suggest. Doing design is a bit easier for iOS, as there is 2 resolutions you need to worry about if iPhone. And another if you going to add iPad support. But all the grunt work is handled by the system. Adding support for tablets, is a lot easier. As it is supported out the box, and you basically just need to add compatibility for different screen sizes, but android SDK also make it very easy.
One thing is, that android uses a filesystem type solution, where iOS uses an actual solution file, which is a lot better to exclude files and add certain files, etc. But the iOS solution folder, can get quite dirty if you do not do it correctly, as a “folder”/group you create in xcode, is not a physical folder on disk.
Also, currently, iOS SDK has no garbage collection, so you need to retain and release memory. Where as android, you do not need to worry about that, unless it comes to using a lot of Bitmaps. But that being said, the new version of iOS 5 that is coming out soon, will be have GC. So it will be a thing of the past.
So, after all that. Android is easier to develop for, but iOS will probably make you more money (as selling/buying apps is not supported in all countries yet). Unless your app becomes hugely popular. I would definitely suggest doing and android and iOS version. I would also suggest learning each platform independently, as apposed to using phonegap or something similar. But if speed is a priority, phonegap can help a lot.
I hope this helps you decide.
Good luck