It depends on what art you're training in and what the focus of the training is geared toward. Street defense vs dojo/tournament techniques are very different in what is acceptable as in above or below the waist. With that said I have always liked the front kick with the ball of the foot if done with intent and target low stomach. The higher you go with the front kick will have it's disadvantages using ball or heel. I prefer the front kick low with the heel and more for a street defense. So for me heel or ball depending on when and where.