I really dislike the look of the yellowing on ABS keycaps. Since I always sit next to a window, both at work as well as at home, I have to take into account that the keyboards are going to practically have sunlight almost directly on them for most of the day. Since I am a big fan of white keycaps, I had to go PBT. When it comes to developing shine, I actually found that on black keycaps it is much more noticeable then on lighter colors. Though not having shine develop fast with PBT is till nice.