>>From the point of view of the application and its requirements, What conditions in do you consider C#/VB.NET a more suitable set of tools?<<

We prefer to use VB.Net when we made datacentric application with heavy database(SQLServer)operations. VB.Net is good enterprise tools. It is not so fast as VFP, but Ken Levy and YAG promisse that will improve its functionality.
C# we prefer to use for development of applications, which heavy use APIs and web applications which require a lot of clientside procesing.