The MultiCAM 4800DX and MultCAM 4800FX use the same basic module but the FX version is more developed and users consistently report better AF speed and lock in with the FX version compared to the DX version.
The features are similar between the two cameras but the reason to use FX for those who do, is because of the inherent characteristics of FX, including the 2.25 times the surface area of the sensor, the 1 stop or more advantage in low light situations and the natural 35mm field of view which also gives a narrower depth of field. For people who shoot portraits, landscapes, night ambient lighting, and architecture, if features were identical, would prefer FX. Those who need more reach and narrower fields of view, deeper depth of field for a given aperture, sports with more modest lenses etc, DX would be a good choice, even if priced the same. There is a place for both and now Nikon has wonderful options in both camps for people who want the traits of both full frame and crop frame cameras. The D7100 and D800 make excellent stable mates and unless a D400 comes out, that will probably be the popular combination for a while. Stan St Petersburg Russia