Whatever fits with the show. A deep male voice describing a magical girl anime would be strange. Similarly, a young female voice describing the latest ultra-violent military flick would be distracting.

I honestly don't mind either way, as long as either gender doesn't have an annoying voice. A+ if the female narrator sounds hot, haha.

It really depends on the content. The voice actor has to fit the show just as the principle actors are cast. Watch and listen to the difference between Daredevil and Kimmy Schmidt on Netflix for a direct comparison. Male and female voices can describe both dark and light stories; there have been wonderful female voices for dark and gritty shows and great male voices for light shows. Just have to give the voice actors the right direction.

I strongly prefer male voices personally (and have stopped watching some things because I found the describer's voice too annoying), but like /u/fastfinge/ said it somewhat depends on the content and your audience.