A friend asked me what the best programming language to use for the web is. I tried to answer the difficult question as objectively as possible.
This is obviously quite subjective, so I will do my best to inform without bias.
If you were just out of college and you wanted to be a programmer I would tell you to choose web applications or mobile (Android/iPhone) programming. Those are the two hottest and largest growth fields--winforms apps are going away. There will be a few client winform apps, but everyone is going to the web.
Obviously you are looking for web technology so I will expand on that.
4. Server-Side Language*
I know that this is a given, but I put this in here to steer you away from technologies like Adobe Flash and Silverlight. HTML5 has absolutely killed them because the browser is now able to play video, sound, and do animation--without any plugins. Microsoft has discontinued Silverlight, Adobe has killed off Flash.
You can see how good your browser is by checking out html5test.com. Check out the possibilities for html5 here.
4. Server-Side Language*
This is going to be the hardest one to answer, because it depends on so many variables. I"ll boil it down to the two best options.
If you want to go with what's hot, what all the startups are using, and you want to build applications quickly with cutting edge technology: Ruby on Rails.
If you want to leverage your C# and .NET background, and you would like to work for a larger company building semi-cutting edge stuff with great tools: C# and ASP.NET MVC4
A few words about both.
Ruby on Rails
Ruby on Rails was built by the guys at 37signals which give us tools like Basecamp and Campfire. Twitter was originally built on Rails until it outgrew the framework (a problem most of us will never have). Rails is built to make you write less code and it was specifically designed for database-driven websites. It has built in support for AJAX and unit tests, and it's support for database changes is unparalleled--no other framework makes working with the database as easy as Rails. Also, it's free to use. You can use your favorite text editor and host your site for free on Amazon's heroku.
On the downside for Rails: it's popular with startups and forward-thinking managers. It's not real popular in the Enterprise. So if you want to log into monster.com and see tons of opportunities, you'll have to search hard in the Omaha area. I just counted 6 jobs on careerlink that mention Rails, 42 that mention ASP.NET. If you lived on the coasts, this would be a great option.
Microsoft has taken notice of the impact and beauty of Ruby on Rails and stolen borrowed a lot of the ideas from Rails and brought them over to the .NET world. :) Your experience with ASP.NET will leverage your history with C# and the .NET framework is very popular in the Omaha area. ASP.NET MVC is quite popular with .NET shops that are trying to modernize their web applications. StackOverflow is actually running on ASP.NET MVC--not Ruby on Rails as many assume.
On the downside: It isn't as cutting edge/sexy as Ruby on Rails. But if I'm going to write .NET code, I really enjoy ASP.NET MVC.
Hope this is helpful!