Well, your dealer may be partially correct about the "Contrast Technology" in terms of the maxmium ratio. Both the 50" & 65" have the same electrical spec expect for PEAK contrast ratio which is 5000:1 for the 50" and 10000:1 for the 65"
Having said that, the typical continous Contrast Ratio for both screens is 3000:1 as quoted in the B&O spec.