There's a brand new study out right this moment by Distimo which shows that apps featured in the Android Market gain massive appeal while apps featured in the iTunes market, either for iPhone or for iPad, saw similar but not nearly as impressive bumps. What you'll see here is two things: when featured in the market, it's actually the iPad apps specifically (when testing iPad apps, iPhone apps, and Android apps on the whole) that gained the most popularity in sales/rating - but it's after the featured period is done where you see the undeniable blast of growth for Android. After the featured period for each of these categories, while iPad apps went up 145% and iPhone apps went up 75%, Android apps went up a whopping 828%!
It's important to note that this whole study was done on the top 100 most popular apps in all categories. Should you have an app that's about to be featured in either market that's brand new, you may well experience severely different effects. You'll see a surprising amount of apps that haven't gained much of anything (the blue category in this chart) for iPhone apps, while the Android and iPad apps have pretty close to equal amounts of more than 100% gain in rank - all of this during the featured period of 7 days. The iPad category then pushes past Android in the most massive gains inside the featured period, that is 200% gains or more.
Another bit of info you've got to consider is how easy or difficult it is to find an application in each of these markets on a regular basis. When you get an application featured in the market, it's also much more likely that blogs and other publications will pick up your app and feature it themselves. What we'd like to see is a featured section for Android tablet apps, also, in the Android Market to see if there'd be some giant amount of differentiation between phone apps and tablet apps like we're seeing with Apple's products.