Cara,
Well the larger size is the main benefit, either to fit more items on the screen, or make the controls bigger for gloved hands, etc.
Other wish list items:
nice thick glass and rigid bezel.
at least 800x600 pixel resolution.
more than 9 colours for 3D screen objects
ability to set it up as a remote screen to another Unitronics PLC (like a PLC-based version of Remote Access - requiring no specific programming). Possibly have a special version with lower cost "hobbled" hardware to support this (eg, no snap or expansion I/O port, less memory, etc, so it can't be used as a stand-alone PLC).