I agree with Chris, from a security perspective neither one of those languages is really ideal because they hide a lot of the internal/low-level stuff that goes on.
But, if those are your only choices, here are a few considerations for picking your poison:
1) Chris is right, from what I've seen VB.NET is used more than C#. Also, when reverse engineering, programs written in VB and/or using the .NET framework are also vastly different than those written in C or C++. So learning VB.NET could help you out down the road if you are interested in RE. VB is also a great language for easily writing Windows apps, but it's not that great if you want to do low-level stuff (my experience with it is limited however).
2) I've never used C# so I don't know how similar it is to C or C++, but I do know that it hides a lot of the low-level stuff that C/C++ don't. However, learning C# could help you pick up C more quickly (which is a good language for a security professional).
So I suppose it all really depends on what you want to do. If in doubt, I'd probably go with Chris's suggestion of VB.NET, but you might want to Google for comparisons between the two languages before you decide.