"Amazon is coming after iTunes. Can it take the lead one day?"
As long as the whole world is concerned, not likely. Not before Amazon expand their worldview to cover more than the continental US states. Apple still have the lead, and don't have to worry about Amazon on the global market.
You can go to the iTunes store all over the world and buy some music or a movie online. Try doing the same in the Amazon store, and you'll end up with an error message, if you happen to live outside the US. Which includes quite a few potential buyers. To lure in them/us, Amazon would have to expand their online stores into the same markets Apple is already covering, and has done so for a good while.