but the duron overclocks better
if you want to overclock you can probably get better perfromance out of the duron 800
If not, the tbird 800 will perfrom better by a significant margin, check out some of Tom's benchmarks, they tell the story.
they're the same "speed". if you're not talking about the altlon classics you'll get better performance out of the athlon. from an athlon classic you'd probably (this is just my guess) get about the same benchmarks as the duron.