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