Quote: "why not just get darkbasicpro it's virtually the same"
Not really.
First, the language for one is Basic and the other is C.
DBPro is based on the same libraries that make up the DarkSDK (at least that is my understanding).
The same program built in DBPro is larger and usually somewhat slower running than that done in C and DarkSDK.
Using C++ and DarkSDK allows you to do really cool stuff with classes (Object Oriented Design/Programming).
It probably comes down to which language you prefer to work in.
Oh yeah, there is a little difference in cost. But if you are serious about selling a game, the initial investment is minimal.
Cheers,
Ancient Lady