I know based off of the reviews, the Pack isn't as firm as Mark because it lacks the core.
How much of a difference is there in the over all squish factor?
I was hoping that there would be an Eden reviewer weighing in, but if you like I can take a stab at this one...
The Pack does have an inner core, and it's the same firm silicone that you see in the Mark. The difference is that the inner core only extends about 1.5" up the shaft, so most of the toy is Supersoft. The Mark, in contrast, has a harder core that goes all the way up the shaft of the toy.
The Pack's abbreviated core means that it's still usable in a harness, but very squishy and flexible. We had a "limited edition" (read: production error) run of the Mikey, Mark & Adam in an all supersoft silicone a year or so ago on our webstore, and this is the closest analogue we've had yet. Very squishy, lots of give in the silicone.....a nicely plush ride.