I also feel a difference in running on Pavement vs trails or grass. But ultimately I dont think it's more or less teh surface but rather the technique or weight?
Some people believe in the match the right gear to the sport thing, and other people just mix the types of cardio up. Here is what I typically end up doing: 1. Swim twice a week/Spinning classes
2. run 2-3 times at 3-6 mile intervals or substitute 2 of those days for either weights(Deadlifts, Lunges, calf ext) or 6 x 40 yard dash.
And take the other days off.