Just go with the T-Bird. The main differnce between the two is size of the L2 cache, 64K for the Duron, and 256K for the T-bird. The faster Durons do not overclock as well as the lower clocked specimens..and with the difference in price minimal, the T-bird will make you a better choice.
Physically the Duron die is 100mm sq. The T-bird is 120mm sq.
They also have different thermal charateristics and voltage requirements.
Peace out..