One difference between demo bindings and regular ones is that some of them can have a significantly higher stand height, which can affect edge-to-edge quickness. They are not all like that, but some can be several millimeters higher, which can make the ski handle and feel different than flat or even rail mounted.
Nordica makes some excellent skis. I haven't skied the Steadfast, although the reviews I have read makes it sound like a ski I would like. I own a pair of hard snow Nordicas (Speedmachine Mach 3), and have rented/demoed several others (all <80 mm). With the exception of the Tempest (which I did not get along with), I really liked all of them. I love how they feel on the snow -- kind of silky smooth, supple and powerful, damp but not dead. Add to that predictable handling, huge sweet spot, great high speed performance, and good build quality, and they have some winners in their lineup.
I think I must be the only one on this forum who actually owns a pair of Nordicas, or has even skied some. They don't get mentioned much here. But me, I'm a big fan.
FWIW, I have never skied a Rossi that I liked. I haven't tried their new stuff in the past couple of years, but I hear they are improving. But their skis up to about 2 to 3 years ago, at least those that I tried, all felt dead and lifeless. No energy, no rebound, no fun. Some were quite stiff, but still felt characterless and boring. Maybe the recent models are better, and some of the Dynastar influence is rubbing off in a good way.
Good luck, and let us know what you end up buying.