Go C#
You should definitely be looking at the Microsoft stack, I have yet to see many bespoke business apps built on anything else. Have a google around to see what skills recruiters are currently looking for... ASP.NET MVC 4 Razor is the hot one currently. That gives you RESTful WCF end points (so you can easily change your front end) and C# for your services. You could even look to a no SQL DB like Mongo running locally or in the cloud (Azure supports it and is now both cheaper and fast than AWS) and keep your data in JSON format and avoid the cost/performance of SQL Server.
Stay clear of OSX development - it is painful as hell! Objective C is a horribly syntax and XCode is a joke - not for professional development. Java is... barely changed since the 90's - still comes with awful tools like Eclipse. Don't be fooled, it is free for a reason!
Nothing compares to VS2012 + Resharper for productivity. Light years ahead of the competition.
Antipathy toward MS comes from the iOS / Android fanboys that have polluted tech websites over the last few years. Non of them are developers, most of them are idiots.
Biting the hand that feeds IT
