There are, so we are told, far more VB.NET developers around than C#
developers (though I haven't actually seen any evidence of this myself, at
least here in the UK). It could be argued that for this reason you may be
more in demand as a C# developer, though conversely of course there could be
fewer jobs to go round. You would need to do some market research on this.
Most companies will want to standardise on one .NET language used within the
organisation, and in my experience (OK, only 2 companies so far) this
standard language is C#, admittedly by default because most developers were
previously C++ rather than VB developers.
On that subject, in general one would suspect that old VB6 developers have
moved to VB.NET, and old C++ developers (who will often have also been
developing using MFC/ATL/COM/Win32 etc.) to C#. Employers may thus have a
preference for C# developers on a generalisation and presumption that they
probably have a larger skill set and breadth of experience. The actual
differences between VB.NET and C# are relatively minimal in terms of their
power, but undoubtedly VB.NET does suffer a lot from a lasting VB6 vs C++
stigma.
Note also that if you are learning the languages, Microsoft are nowadays
moving towards doing their code examples in VB.NET because of the supposed
larger number of VB.NET developers:
http://blogs.msdn.com/tomarcher/archive/2005/11/21/495282.aspx