Which technology to be used and when

As Mahesh intimated, the two languages have virtually the same capabilities and the same execution speed so it's a matter of taste which you use. 


However, there's one thing which C# has which VB.Net doesn't have, namely the ability to use pointers in unsafe code. This is useful for accessing memory mapped devices (e.g. embedded programming) or speeding up some types of algorithm (e.g. certain games).


Personally, I also find it much easier to use C# rather than VB in applications which require considerable interop with unmanaged code written in C/C++ because the syntax of C# is, of course, closer to the latter.

Both are same. It's your choice. I go consult at some companies. At some place they want me to use C# and others wants me to use VB.NET.