The first aim of the present paper is to compare various sub-Riemannian structures over the three dimensional sphere S^3 originating from different constructions. Namely, we describe the sub-Riemannian geometry of S^3 arising through its right action as a Lie group over itself, the one inherited from the natural complex structure of the open unit ball in \mathbb{C}^2 and the geometry that appears when it is considered as a principal S^1 -bundle via the Hopf map. The main result of this comparison is that in fact those three structures coincide. We present two bracket generating distributions for the seven dimensional sphere S^7 of step 2 with ranks 6 and 4. The second one yields to a sub-Riemannian structure for S^7 that is not widely present in the literature until now. One of the distributions can be obtained by considering the CR geometry of S^7 inherited from the natural complex structure of the open unit ball in \mathbb{C}^4 . The other one originates from the quaternionic analogous of the Hopf map.