There was a big difference between VB 6.0 and C++, but the difference between VB .NET and C# is negligible. VB 6.0 was a completely different language with different libraries when you compare it to C++. Now, VB .NET and C# are essentially layers on top of Common Intermediate Language (CIL aka MSIL). That being said, the .NET languages use the same libraries and the only difference is "syntactical sugar" (see the comparison link below).
That's my quick intro...now on to answering your questions...
I don't think that one is easier to learn than the other, but if you learn one you should be able to learn the other in under and hour (not the same claim can be made with VB 6.0 vs C++).
For marketability, the simple thing to do is run a quick search on a job board and see which keyword pulls up the best jobs. I think you'll find that C# pulls up more jobs than VB .NET. However, if you understand both anyway it won't matter.
I use and teach C# 99.999% of the time, but I have the occasional client that prefers VB .NET, and trust me - the switch is incredibly easy to do.
The link below is a VB .NET vs C# comparison. You'll notice that the comparison chart is just syntax - not architecture, functionality, libraries, platform, extensibility, etc.