I haven't used it yet but the Bing Maps API is pretty damn cool looking and they have an online tool to mess with the API code and see the outcome too. Still debating on which one to use for a project I am working on, my partner is very pro Google but I've heard horror stories about their enterprise model when it comes to cost.
Google has more outside support for help with the API, Microsoft has more in house support for Bing's API. The biggest deciding factor for me so far is I like the look of the Google maps better.
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