I have used Android extensively, and just picked up an iPhone 4 yesterday (I had an original 2G and 3G).
You can buy MP3s from Amazon MP3, and use the MP3 downloader Amazon provides to automatically load them into iTunes. Works flawlessly, the music is not tied to your one computer or one single account.
As far as the navigation app, there is no free equivalent to Google Maps Navigation on the iPhone. I will adjust, but I miss that.
iOS is a better experience, IMHO. Android has its strengths, but if you pick up a different Android phone, you could be in for a completely different user experience. iOS is iOS on each and every iPhone you pick up. There's something to be said for that user experience. Also, media management on the iPhone is infinitely easier than Android.