I have 50, 90 and 180 lenses. All perform to a consistently high standard (the 50 has very slight barrel distortion). As you note yourself, the 180 is a general-purpose lens (which I find great for portraits) while the 150 has been designed rather more with portraits in mind. With the 150 you will tend to work slightly closer to the subject and thus obtain slightly more rounded drawing - which is of course entirely a matter of personal taste.