The second equation shows that the kinetic energy of an object depends directly on its mass. And kinetic energy most certainly is extremely relevant, in that it is a measure of how much damage a moving object is capable of doing.
The first equation shows that momentum is also linearly dependent on mass. That is also relevant, but in a more subtle way. A collision between between a massive object and a light object results in a large change in velocity of the light object, and a small change to the massive object. Peak acceleration on the light object are therefore much larger - thus more damage will be wrought. Which is why a pedestrian is very much worse off when in collision with a 4x4 SUV than, say, a cyclist when both are travelling at the same velocity.