I would consider what everyone else uses - not the folks on this site, but the folks who write the same kind of massively parallel, or super high-performance applications.
I find they all write their code in C/C++. So, just for this fact alone (ie. regardless of any speed issues between the languages), I would go with C/C++. The tools they use and have developed will be of much more use to you if you're writing in the same language.
Aside from that, I've found C# apps to have somewhat less than optimal performance in some areas, multithreading is one. .NET will try to keep you safe from thread problems (probably a good thing in most cases), but this will cause your specific case problems (to test: try writing a simple loop that accesses a shared object using lots of threads. Run that on a single core PC and you get better performance than if you run it on a multiple core box - .net is adding its own locks to make sure you don't muck it up)(I used Jon Skeet's singleton benchmark. The static lock on took 1.5sec on my old laptop, 8.5s on my superfast desktop, the lock version is even worse, try it yourself)
The next point is that with C you tend to access memory and data directly - nothing gets in the way, with C#/Java you will use some of the many classes that are provided. These will be good in the general case, but you're after the best, most efficient way to access this (which, for your case is a big deal with multi-terabytes of data, those classes were not designed with those datasets in mind, they were designed for the common cases everyone else uses), so again, you would be safer using C for this - you'll never get the GC getting clogged up by a class that creates new strings internally when you read a couple of terabytes of data if you write it in C!
So it may appear that C#/Java can give you benefits over a native application, but I think you'll find those benefits are only realised for the kind of line-of-business applications that are commonly written.
