There are a few capabilities of Google Maps for Android that don't exist on the iPhone version of Google maps -- specifically StreetView and Compass (a unique and very compelling feature). There are rumors that StreetView is coming to the iPhone. But Google, so far, has reserved the richest experience of Google Maps for its own Android platform.
Here's a Google Mobile Blog post that details the full range of features and capabilities of Maps for Android; and here's a video that demonstrates these features.

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Related: T-Mobile has now put an "Android emulator" online here.