Which is generally accepted to be parrallel 250's parrallel 4/0 AL is 340 amps, 500 copper is 385 amps and parrallel 250's is 410 amps. As to the OP. Assuming that the spare conduits are required for both copper and aluminum, the 250 aluminum would be significantly cheaper (about 40%) and the cost flucuation history is much more stable.
Read the OP again, he says " 4#500kcmil-4"c versus 170 ft. of parallel 4#250kcmil-3"c"
Yet he says only 100 feet of 500's and 170 feet of the 250's:?
I would guess he wants to compare single run of 500 copper vs parallel run of 250 aluminum.
I think price of copper alone will be much higher than the aluminum alone. Copper is harder and heavier to handle, aluminum will be two pulls but is easier to handle.