Gable designed verandahs ultimately give you an increased feel of space and openness compared to flat roof ones. Complimented by the right choice of roof covering, allowing for natural light, the area will seem larger.
Materials comes down to a personal preference, while metal verandahs have a miss conception of being lower maintenance, timber has the opposite miss conception of needing more maintenance. Wider and larger spans are better done with metal, while complicated and more detailed designs are better done with timber.
Decide on a design option first and then chose the more suitable material.