DBPro is a great and powerful language and a lot can be done with it beyond a lot of other basic programming languages. It easier to learn and understand than c++ and so much quicker to develop games, programs and software with.
However c++ opens up a whole new world of programming because of its power. It takes a long time to learn, and even then you have to teach yourself more along the way. When you are writing your code, you are prone to typos and silly mistakes that you don't get in DBPro.
Also using c++ with the GDK has really only a slight increase in speed of your programs, but on the plus side, you can write in c++ intense calculation code.
I would say to anyone, if you do not know c++ or still on the beginning learning curve level then start with DBPro. It has all the power of the direct x libraries and gets you in good stead. A lot of excellent software has been written in DBPro so no-one should knock it in anyway.
Warning! May contain Nuts!