A.C. vs. D.C. traction motors   refers to the electrical transmission:

D.C.:  The engine drives an A.C. main generator, which generates A.C. current rectified to D.C. by diodes that powers D.C. traction motors.  The advantage is significantly lower initial cost.  Disadvantages are higher maintenance costs for the traction motors, less tractive effort at low speeds (less than ~ 11 mph).

A.C.:  The engine drives the same A.C. main generator, which generates A.C. current rectified to D.C. by diodes.  From that point it differs: the D.C. current is converted to A.C. which powers A.C. traction motors.  The advantages and disadvantages are opposite a D.C. locomotive. (You might be wondering why the A.C. locomotive goes from A.C. to D.C. and back to A.C.; the answer is that the D.C. rectification provides "clean" current that the invertors can use to create the proper frequency needed by the traction motors. The reason the main generator is A.C. and not D.C. is that D.C. generators are too large, too expensive, and too complex once horsepower climbs past 2,000 or so.)

North American railroads such as BNSF, UP, and KCS have determined that A.C. is more cost-effective for heavy-haul trains such as coal, and D.C. more cost-effective for all other trains.  NS has favored D.C. for everything and CSX A.C. for everything.  The decision is based on the physical characteristics of the railroad, the operating plan, and the traffic mix.

The only difference in the way A.C. and D.C. run is that a train powered with A.C. locomotives can have a very low horsepower-per-ton ratio and still move without exceeding the heating limits on the traction motors.  A.C. motors in fact can run to a stall at full current without damaging the motors, whereas D.C. motors will quickly overheat at low speeds and full current and experience fatal damage. 

The "e" in SD70ACe stands for "enhanced'; it is a major redesign of the previous SD70MAC but shares the same basic specifications, horsepower, and prime mover. 

AC vs DC traction motors which one? 

In engineering terms, every device, invention, or product comes with its merits and demerits.

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