My understanding, Poly_RPO resistor process varition is bigger ther NW resistor. But, NW resistor will take more area larger than poly resistor for min. rule. BTW, the temperature coefficient of ploy resistor might be opposite to NW resistor.
You must take into account: min width of Nwell res is much bigger than min width of P+Poly res. And the min space of Nwell res is also much bigger than min space of P+Poly res in design rule.
For final conclusion, all of these must be considered.