Wow, I can't believe people really believe that mass makes a big difference in free fall motion. Galileo demonstrated this years ago.
http://www.grc.nasa.gov/WWW/k-12/airplane/ffall.html
The only thing that could affect the motion would be air resistance, which is largely related to the shape/surface area of the object (hence why a feather takes longer to fall to the ground than a ball). But for the human and the squirrel, considering they are both falling from such a small distance, air resistance is negligible.
Now to answer the first part of your question, the human will most certainly feel more force exerted upon him/her. It's simple Newtonian physics - every action has an equal and opposite instantaneous reaction. Both the human and the squirrel fall according to Newton's 2nd law - the equation 'F=ma'. The value 'a' is constant for both - however, the mass of the human is much greater than the mass of the squirrel. Hence, the human exerts a greater force on the floor (this force is a consequence of mainly gravity). As a result of Newton's 3rd law, the floor exerts this exact same force to the human, which happens to be much greater than the force that would be exerted on the squirrel.
Now, whether or not the human feels more pain or not is entirely up to his/her strength/tolerance for pain.
However, a very good question of yours is, "Why does falling on concrete 'hurt' more than falling on grass?"
This actually has to do with momentum. Now for both surfaces, your change in momentum (which is the product of mass and velocity) will not change. However, as concrete is a much harder surface than grass, the time you are in contact with the concrete will be less than the time you are in contact with the grass.
I'm sure you know the following variation of Newton's 2nd law, being F=delta p/delta t, where F is the net force, delta p is the change in momentum (impulse), and delta t is the change in time. We can rearrange this equation to give the following: delta p = F*delta t. As you can see, since we know that the change in momentum is constant (delta p), that means that if we were to lower the value of delta t, the value of F would have to increase as a result of this. Hence, F would be larger for a concrete surface, where the time of contact is smaller than that of grass. As a result of this larger force being exerted on the ground, the ground will have to exert the same force onto the falling object, in accordance with Newton's 3rd law.