I read both manuals. The Z7 is larger and heavier, which suggests to me that it has better internal parts -- or at least different parts. The external hardware (speaker posts) are better in the Z7. The Z7 has an extra HDMI front panel connection.
The most significant difference is that the Z7 allows users to adjust the video signal, using the Anchor Bay chip, for Mosquito Noise Reduction, Block Noise Reduction, Detail Enhancement, Edge Enhancement, Brightness, Contrast, and Saturation, while the 3900 does not.
The discussion of this difference is on pages 100-101 of the Z7 manual.