I find the following to be true most of the time:
DD coils have more of a half football fov than a knife blade
DD coils separate better in one direction- when target to trash is is in a line from left to right
DD coils work better in high mineralization
CC coils go deeper but swings need to be overlapped by 1/3 of coil diameter
CC coils can separate deep targets better
CC coils ID more accurately- bottle caps etc
CC coils detect coins on edge better when swinging over the coins from edge to edge