Being able to connect your camera to a computer and view photos in real time is important for many different professional applications. Fortunately, both cameras support tethering (live view is only available on the D800E, however).
Since our 2011 review of the 645D, Pentax has launched proprietary tethering software for the 645D (Pentax Image Transmitter - $249). PK_tether, a community-built tethering application, is also supported, and it actually has more features that the official software. The D800E supports proprietary tethering software from Nikon (Nikon Camera Control Pro 2.0- $146), which can also be used with virtually any other Nikon DSLR.