Depends on the deck, but I believe Dual Ball to be superior. Great Ball is a guaranteed basic whereas Dual Ball is a bit more flippy. Doing the math Dual Ball has a 50% chance of getting you one basic, 25% chance of two and a 25% chance of none. If you like the odds use Dual Ball, otherwise Great Ball is more "reliable".
If you need Pokemon that do not have play-from-hand powers, and want to guarantee one, Great Ball. If you don't mind playing a Trainer that has a chance of doing nothing, or need the play-from-hand power, Dual Ball.
Even before I knew that, I was still slightly more into Dual Ball. But the whole idea of this thread was to get other peoples opinions to make the better choice.
Like Dennis Hawk said, Great Ball is better than Dual Ball in Scizor/Cherrim, as you don't run powers (if any, you play Q's and R's). But Dual Ball, though it could possibly do nothing,can get Uxies.