Should I use SQL or NoSQL for this specific design (surveys)?
Typically No-SQL solutions were implemented due to massive scale and not needing 100% ACID compliance.  Flattening data out and de-normalizing it for performance gains were critical in getting us to where we are today with solutions such as GFS, Hadoop, Mongo, Basho RIAK, etc.  

Your case doesn't scream No-SQL, if anything you'd probably be better served with MS SQL server since you're using the ASP.NET stack and MS SQL Server provides a lot of functionality with ASP.NET through CLR, powershell (which can also call .Net objects) and playing very nice with IIS (I assume you'll also be hosting it on IIS).  This also sounds like something you can use SQL Server 2012 Express thus saving licensing costs.

MS SQL Server will also let you easily 'join' data across non related tables in cases where you come up with a new idea, and on a No-SQL solution that would be more difficult to implement.

But in reality, looking at your requirements and what you're going to be using it for.  You can go either way.  If it makes you happier to use a No-SQL solution and get your feet wet, awesome!  Go for it!  End result probably won't matter, but to strictly answer the question, which is better for a .Net app with the usage patterns you described, I'd personally go with SQL 2012 Express.

I'm curious to see what others think!