The difference in pressure on the 2 sides of the wad is what pushes it.

If you use a vacuum, your high pressure side is roughly 15 psi and the other is maybe 0 (probably more, but whatever). So the max difference is 15 psi.

If you use pressure, the low pressure side is 15 psi, but the high side can now be whatever your compressor gives. Maybe 90 psi or more. So the pressure difference is much higher.

When pushing long distances where there will be more drag due to the line being pulled, the higher pressure difference will help more.

While the air is "stretchy," that's not really what matters.