The question asked was about the differences between the two software systems in the past.
From that perspective the two systems had very different objectives and target audiences.
Interlisp-D was focused on ease of use, the GUI, incremental development by researchers who weren't focused on programming per se. This explains the features around the GUI, error correction, programmer's assistant, source file management. Interlisp was forgiving.
I always imagined Symbolics Genera was designed for the expert user who wanted the most power, speed, capability possible.
Race car to off-track vehicle.
As for hardware, the Xerox D-machines weren't particuarly designed for Lisp. Comparing the hardware is kind of irrelevant.
Today, medley runs 1000 times faster and a much larger address space, so there's enough room to try things.
The Medley Interlisp project includes many "help wanted" items in terms of documentation, testing, Common Lisp compatibility, and things relating to adapting to modern hardware (keyboard, mouse, display) and software (OS, installer, unicode, etc) as well as finishing some "work in progress"items that were done after Medley 2.0. We started with Medley 3.5 to get the larger address space (4 bits) and symbol space.