25mph => 45mph : 1400 "units" increase
65mph => 100mph : 5775 "units" increase
So at least in terms of kinetic energy and therefore destructive potential, 100mph on the freeway is more dramatic than 40mph in a residential zone.
A car that weighs more is safer because its velocity changes are slower (barring collision with an immovable object) and because it will usually have better crumple zones that absorb more energy before turning the passenger compartment into a compactor.
If higher kinetic energy or momentum caused higher lethality, heavier people would die more often and/or heavier cars would be more lethal (for its own occupants) in wrecks at the same speed.
Since it's not always possible to know what speed a car was going when it crashed, and since I haven't found any tables or graphs of either speed or KE [or momentum for that matter] vs lethality, I doubt there's enough data to draw any conclusions about KE being better at predicting lethality. I don't understand what the focus on collision energy is for.
Crumple zones can be independent of weight. A lighter car is more able to stop (less energy for brakes to dissipate) or avoid a crash (can sustain higher cornering forces, tires being the same).
100 vs. 65 mph on freeway: you certainly die vs. you probably die, depending on the accident. Freeway accidents are very bad at either speed. Luckily, there shouldn't be any pedestrians, children running into the street, or bicyclists.
The danger of freeway speed has more to do with differentials. Accident risk goes up 2x every 15-20mph of raw speed but 2x every 3 mph of speed differential (in either direction). If you're in traffic and driving 100 mph, you're probably an asshole because most people won't be, but it's the speed variance (and weaving, tailgating, and other asshole behaviors) that makes it dangerous.
The survival rate of a 40mph collision is around 50%. The survival rate of a 80mph collision is 0%.
There is a _big_ difference between a 60mph cruising speed and 100mph cruising speed.