Who burns the more calories?
For exercising as part of a weight management goal, the workouts that burn more calories are more effective. In running, distance and runner's weight are the biggest factors in determining calories consumed. Difference in speed has only a tiny effect. However, walking is more efficient than running, so caloric consumption of running and walking have to be calculated separately, and the two added.
By the way, to tone or increase bulk of ab muscles, you need to do strength training. Running won't do it.