Many years ago I purchased a number of metal frames, having been influenced by the fashion or trend of the day. I quickly got tired of them at least for my exhibition prints and just used them to frame photographs to hang at home. Although I have been careful with them when moving them they do not stand up well and are now being confined to the dump for they are really badly marked. On the other hand I have about 100 wooden frames, light ash and American oak which is light brown, and despite having hung several exhibtions and sent them to a number of galleries they are still in very good condition. If there is a small mark it can be rubbed out on wood but not on metal. For that reason alone I would now always go for wood.
Choosing frames is very personal and aesthetics have to be considered so I hesitate to insist that one is better than the other for your work. I relate my experience mainly to point out one of the likely long term problems with metal frames. I have had both colour and black and white photographs on my wall in wood frames for many years and have noticed no deterioration at all.