I used both for a while, and I liked the 955 better. The images were clearer and I had an easier time calibrating the colors. However, since I run my screens at 1280x960. the 955 could only provide (officially) a refresh rate of 75 Hz which is a bit low for me but might not matter to others.
My personally feeling on these monitors was that they were bargains half a year ago. Yet now, their price still hasn't changed much while the other brands have dropped quite a bit. I would go with the KDS or Hitachi now since they're better in quality and about the same price (under $300). The only Samsung worth getting is their 900nf which is still one of the best 19" monitors around.