i would have to say the duron will run better on most apps. That is if you are refering to the classic athlon and probably even if you are refering to the T-bird. From what i understand, a duron 700 runs just about as fast as a classic athlon 700, unless the prog is really cache intensive.
If you are talking T-Bird, I would go with the T-Bird.
If you are talking Athlon classic, I would go with the Duron.
I think the T-Bird, even at a slower clock speed has a big advantage over the Duron. Its only 200MHz.