At the risk of sounding like a non-committal politician, there are good and bad examples of each.
To my ear, and with the right ancillary equipment, the two best reproducers of voice and strings are the Quad 57, and the Cogent field coil midrange compression driver. There are of course also very high quality midrange cones; the berylium coaxial driver in the new Andrew Jones-designed TAD speakers is nothing short of superb, as is the driver in the YG Acoustics line. In my opinion, the two latter speakers have significantly raised the bar for low distortion drivers. Both are of course, very expensive.
On the other hand, I have heard ribbons and planar magnetics whose distortions made me want to leave the room.
The bottom line is, the degree of variation within any type of driver renders any generalization pretty much meaningless. As such, you need to hear the drivers that intrigue you to decide for yourself which you like (keeping in mind that how they are implemented is critically important).
Larry
Edited for clarity.