I have the 950p and my mother has the 955df. The 950p is not flat, but not excessively convex either. It is a great monitor. The 955df appears to have a better picture because it is flat, but I think they both are great. The one thing that bothers some people about these monitors is that around the edges it is very hard to not have the picture bend ever so slightly (regardless of your tinkering ability

). It is a non-issue for me and my mother. However, some people are perfectionists. Perfectionists shouldn't buy inexpensive $200 monitors,...and should stick with the more expensive stuff.
I know nothing of the 950b though...