Does WRF simulation use sigma or eta vertical coordinate system?
WRF is using the sigma (terrain-following) vertical coordinate. However, as @gansub has already referred, in WRF V3.9 you can now select a hybrid sigma-pressure vertical coordinate. The advantage of this is that the coordinate is terrain-following near the surface, but it 'converts' to pressure levels at higher levels, which improves the accuracy of the calculation of the pressure gradient. Over very steep terrain the sigma coordinate extends this steepness throughout the atmosphere which is not very ideal.

For sigma coordinates, you still define the eta-levels. (Note that it sigma coordinates is just a normalisation of pressure)
$$
\sigma = \frac{p-p_t}{p_s-p_t}
$$
where $p$ is specified through
$$
p=A+Bp_s.
$$
It has been a few years since I played around with WRF, so I don't know how this change affects computational efficiency. If you get any results on this that will be interesting to hear.