NOTE: If this helps my web app is coded in node.js
They have an AWS-like virtual machine offering as well as the heroku-like hosting, the service is about as reliable, and if you set up your entity now you can register for bizspark and get a ton of free service to get you off the ground.
Previous discussion: http://news.ycombinator.com/item?id=5293098
Currently, I use Azure for my projects. Primary reasons are due to being a C# developer professionally, and it just made sense to use it. In Visual Studio, it is very easy to publish projects directly to Azure without ever actually logging into the Azure dashboard.
I can't comment on price comparisons, which are much different. But I'd say give Azure a try for free initially to demo it and see if it fits what you need.
One thing of note that I feel is not mentioned often when discussing these two options. If you are planning on using Azure's storage options, there will be a sort of "vendor-lock-in" involved with the code-base. I have great experiences with it, but both Azure and AWS have different approaches to how they handle storage options including the NoSQL choices such as Azure Table Storage.
Additionally, I have used Nodejs on Azure. I truly enjoyed it when using Azure Table Storage for my database. Overall publishing to Azure was pretty simple and required minimal setup effort. Let me know if you have any questions!
If that's the case, I would suggest AWS or something like Linode. While Azure has Linux support, I think you'll probably be better off with a service where the majority of users are using Linux and they have more supported distributions. AWS is probably a better fit there.
If you're writing your node app on Windows, then I would undoubtedly go with Azure since you'll be able to utilize Microsoft support and will likely need it at some point.
This is the same problem that Python, Ruby, and PHP devs had to deal with for years. And it's one of the main reasons that most of us write code on Macs when we deploy to Linux since it minimizes the cross platform headaches (although it doesn't fully eliminate them).
So long story short, if you're developing on Windows, I would recommend sticking to all things Microsoft, including Azure. You're more likely to find solutions to your cross platform problems, or at the very least have an easy path to move to Windows if they become a nightmare.
AWS only has your 3rd requirement (scalability). If you care about the first two, you'll have to build that yourself using the API's to handle the random failures from AWS. I've never used Azure so I'm not sure on that side.
If price really doesn't matter (over the other things) you probably want something like Rackspace's Storm On Demand servers.