Probably depends on how you run. If you're a forefoot striker, running on asphalt is probably better than running on grass, because the vast majority of the impact will be absorbed by the calf/foot muscles. The danger from stepping in a hole or tripping over an object hidden in the grass probably outweighs the minimal increase in impact from running on concrete (not that either of these should actually be a significant danger).

On the other hand, if you're a heel striker, the reduction in impact might be worth it. Stomping on concrete with your heels sends a lot of force through your ankles and knees.