1) High current capability of diffusion resistor in comparison with poly resistor.
2) Diff. resistor has an feature, it can limit current due to velosity saturation effect. So max. current is depended on the resistor cross section.
3) High resistance in signal path gives u increased delay for signals.
Diffusion resistor can spread heat very quickly while poly resistor can not since oxide is arround poly. Poly resistor is more easy to be burnt than diffusion resistor at ESD zapping.
I agree the point lianggu said. poly resistor is more easy to be demolished by the ESD thermal. We know a ESD device must protect itself from ESD current first,and then it can protects others more.By the way,N-well resistor welcomed in ESD.
Diffusion resistor can spread heat very quickly while poly resistor can not since oxide is arround poly. Poly resistor is more easy to be burnt than diffusion resistor at ESD zapping.