C# Vs C++

Question

I'm beginning a new application that will have the following features/functions:
- runs on Windows 2003 server
- web based interface
- read/write to tape libraries (probably through RSM)
- raw reading of large (1TB to 2TB) disk volumes (block reads)

I've had advice from a consultant that I could not readily do all of this with only C#. He was adament that some components would be far easier in C++ than in C#. My preference is to write the app in only one language - preferably C#. I started writting the app with consultants help - he using C++ and me using C#. The C# code was fairly straight forward user interface stuff (and the prototype app was only a windows app - not web). The C++ code seemed convoluted and overly complex. Is there a consensus on what should not be done in C# or what should only be done in C++?