Heel running is easier if it's what you're used to, due to muscle memory and different conditioning of muscle groups. Your leg adapts to the stride it's used to, so you could just as easily say that heel-runners have atrophied lower legs as that toe-runners have strong ones. I would say heel-running is more intuitive for a longer stride and a vertical posture, and less so for a short sprint. Whether heel-running easier in a vacuum is not "undeniable," mostly due to the lack of such vacuums (vacua?).