Rijndael S-boxes: Where do the $\mu$ and $\nu$ polynomial ring elements come from?
Finally I've went to the source and I've mailed the Rijndael's authors. They have answered very fast and very nice.

I've understood the other way around. The affine transformation is over the vector space $((GF(2))^8$ and what they've say as simplicity was that, between all the possible affine transformations they select one that can also be described as product and addition in the ring defined by modulo $z^8+1$.

There is no mathematical argument to select as $\mu(z)$ any of the invertible elements above the others. And for the $\nu(z)$ selection something similar happens but here exist the restriction to avoid any fixed or opposite fixed points.

Then the only argument to choose one or another can be the calculation time, but once the S-Boxes are pre-calculated it doesn't provides any extra advantage.