in addition to Cattle Rustlers comment
Dark GDK .Net - Tends to be a bit easier - over C++. Additionally there are people on here writing little helper things to speed development in .Net which is already faster than C++ due to its ease of use.
Additionally - the DarkGDK .Net AFAIK has one of the developers on here a lot - something I don't usually see for the C++ developers in the forums - HOWEVER - there is just as much support for both - due to some of the moderators on here and some users that are simply loaded with good "techy" info when you need a hand.
The DarkGDK (C++ version) is inherently faster however - because there is less software bloat - which .Net does suffer from. You pay a price - one way or the other.
First - both as far as I can tell - are very good products.
Neither is perfect.
.Net version is easier to write code for than C++ but for the most horsepower - C++ has much less overhead than C#.
all this is pretty much talked about in various threads on here and I'm only repeating it for your convienance - but you might want to give these forums a good read and see what else you may find out that could help you with your decision.
I went for the C++ - for awhile - I started to regret it - as I don't code C++ everyday - far from it - otherwise. I do use C# at work. But once I dealt with the learning curve - and started to get the hang of it - I am VERY VERY happy I chose the C++ - because I have more CPU power available to me - and as I'm getting better at my 3d coding and C++ in general - C++ is almost as easy as the C# with the speed. A few more lines of code here and there - but worth it in my opinion.
Also there is no need for an external Licencing validator app with the C++ version - HOWEVER in all fairness - the .Net crew at TGC is going to make that go away if possible - (they are working on it now) - So if C# is really your cup-of-tea - then perhaps you get .Net and that little thorn will go away.