I prefer brass. I have an old 70s Cimar 12 string made by Ibanez that I play as a 6 string. Noticed one day the plastic saddle was broken in the middle. First I made an aluminium saddle - very light, but quite metallic in sound. I then made a brass one - has much better, balanced treble and bass response. I then installed a compensated bone saddle - very bass loaded sound. Really emphasises the wound strings, can barely hear the unwound treble strings.
Swapped the saddles many times before settling on brass. Maybe because a 12 string has heavier construction it isn't as bright? Don't know, but I prefer the brass after comparing plastic, aluminium, brass and bone to each other.
