955SL is the cheapest, it's a plain 19in shadow mask monitor (not flat)
955DF is a shadow mask also, but it's flat in exchange of lower refresh rates
900IFT is a shadow mask, flat like the 955DF, but better quality (higher res. and refresh rates)
900NF is the best IMO, flat aperture grill, better color (but it has the two wires that all aperture grill monitors have)
The best value is the 955DF, it's perfect if you don't use high resolutions. If you have the money, go for the 900nf, it's a much nicer monitor. You should get a flat monitor, so that leaves the 955sl out, and the 900ift is good, but since it's the same price as the 900NF, I'd go for an aperture grill instead of a shadow mask.(unless the wires bother you)