It doesn’t matter what is going to be around in 2015. Nobody can predict future but you shouldn’t buy into proprietary platforms that are offered by single vendor.
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Amazon EC2 is good candidate to deploy your application on because Amazon compatible services can be easily provided by other companies as well. Even if you will want to host your application on your own servers at some point, it won’t be a big problem. Amazon EC2 is practically zero lock-in.
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Google App Engine is just bad candidate because there is not going to be compatible product from other company for very long time if ever. It’s just too proprietary and Google doesn’t plan to release their technology. To me it’s 100% lock-in and if you decide to move somewhere else, it will be impossible without massive rewrite. I would be very surprised if any ambitious and big project would bet on GAE.
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Windows Azure is not looking as bad as Google App Engine. Although it’s still going to be hosted exclusively by Microsoft, it might be possible for other companies to come up with (almost) compatible cloud service. After all, core pieces of Windows Azure are based on well known SQL Server, IIS and .NET framework stack.