They used to say in 35mm that 90 to 105 gave the least distortion to the face. I always found 135 to flatten the nose. anything under eighty you wind up in their face and can be intimidating.
A lot of portrait is more than just the equipment you shoot. You have to be concerned with how the subject reacts to being photographed as well as what the image looks like. If you shoot with a very sharp lens you might want to think about a number one difuser or even more.
The nikon lens and probably the cannon as well can play hell with facial pores ect.
Just some random thoughts