I'm gonna recommend .NET here, mainly because that's what I use and like, but my top reason would be flexibility.

You don't have to use only one language, use many. If you already know C# you can start with that. If you get tired of it you can pick up F# to write algorithms and calculations. Want to learn Python? Get IronPython and use it to write Unit Tests or a rich interface with Silverlight. You name it, everything under the same platform.

If there isn't a requirement that prevents you from using .NET, I don't see a reason not to use it.