You will be able to get the Athlon to higher clock speeds if you get a fast one to begin with (1 Ghz+), but the Duron will probably overclock better. It's fairly common to get any speed Duron to 1 Ghz, however that is about the limit. Athlons will usually overclock about 200-300 Mhz from stock with no upper limit. People have reported 1.6 Ghz with an air-cooled 1.33 Ghz Athlon.
It's up to you:
Duron - better overclock
Athlon - higher clock speed
The Duron is probably the better choice. Why? Well, if you fry the chip then it's not real expensive to replace it. But, it all depends on what clock speeds you want to get to. I haven't see a Duron go significantly past 1Ghz. Athlon's will go all way to 1.6Ghz (depending on the chip).
Info on overclocking the T-Bird can be found in Tom's CPU Guide.
Don't get me wrong here...I dig Tom's but Anand had the clearest instructions I found. That will help a lot but playing with your BIOS or jumpers is the best way to figure it out. Trial and error is the best teacher...Just make sure you know how to reset the board and that you've got an okay HSF.
Oh, and I'd say go for the Duron. If your goal is 1ghz then get the 750 version. Then you can just set the Bus at 133 and you're there. If it runs stable at the frequency you can mess around with the Multiplier to get the clock higher. I got an 850 TB and quickly learned that I should've gotten a 750 and save some money. It won't run much above 1050mhz and 1000mhz was my goal anyway.