.....I only have the 100mm focal length. For portraits I think I'd like to have a 200 but the 200 is more expensive, heavier and slower than a 150.
I'd go for the 200 (best perspective for close portraits), my ideal portrait lens is the 180mm for 6x7 format (different camera), I use a 250 for close portraits. Remember, the expression the subject gives you is sometimes a function of how close you are with the camera/lens. Close is intimidating; setting back further is more comfortable for the subject being photographed.
"Pictures are not incidental frills to a text; they are essences of our distinctive way of knowing." Stephen J. Gould