I'll start by saying that I don't fall into either camp, but I can see advantages to both bullet mass and velocity.
But for those who like bullet mass - here's a simple experiment. Line up five or six quarters on a smooth table so that they are all touching. Press your finger on one quarter and slide it away. Then send it smashing into the line of quarters with as much or as little velocity as you wish (you have to take your finger off just before the quarter collides with the others). You'll notice that no matter the velocity, one quarter goes flying off the other end. Velocity at impact only seems to govern how far the other quarter goes.
Now send two quarters smashing into the end of the line. You have to hold the two quarters so that they are together when they collide - as if they were one unit. Now you'll get TWO quarters flying off the other end. Again altering velocity will not affect the number of quarters flying off the lineup.
So, the answer is both mass and velocity matter. In such collisions increasing mass and velocity have an effect, BUT most importantly the effects cannot be substituted.
