Q - DarkBASIC Pro is new. DarkBASIC is old. Then why should I use the older version?
A - Because both are quite different languages! DarkBASIC Pro isn't actually some pro edition of DarkBASIC but its quite a variant of DarkBASIC Classic.
Game Development is a thing generally not taught in class rooms. You have to learn it your self. It means you have to read books and tutorials.
Now I don't think anybody would like to spend money on books for easy tool like DB! I myself would prefer browsing and using online free tutorials available to learn this thing.
Now here's the catch. 80% of code or tutorial you will stumble upon while surfing the net will be for DarkBASIC classic. You download something cool, run it in DBPro, and OOPS! ERROR! it says. Ultimately you will end up hating the language. Now you don't want to do that, do you?
Another reason is stability. In race of adding more functionality to DBPro, the stability has suffered. So if you want to try some of the new things, give pro a try, but if you want solid stable game, classic is the way to go.
Now another reason, which is more of a personal opinion rather than real reason, but let me share my feelings with you! It is the IDE. The first thing I was attracted to is the IDE. DBPro has so application-like-look, it just doesn't appeal to me. When I installed DB the first time (some years ago) I didn't know a thing about programming. But when I started making game I remembered that DarkBASIC is the thing to go for. And the part I really remembered was its GAME-LIKE interface. When you are making a game, you want your environment to be game-like too! But of course thats my opinion!
The main reason is, if you start learning with DBPro, you will require to put more efforts than you would have to had you chosen DarkBASIC.
SO FOLLOW MY ADVICE! GO FOR DARKBASIC!