I suspect it will be hard to find anyone who uses both a mirrorless camera and a DSLR side-by-side under identical conditions, let alone someone who can give you a quantitative answer.

Common sense would lead me to believe the same amount of dust will enter both types of camera during lens changes, the only difference being that with the mirrorless camera it will fall directly onto the sensor, rather than mainly onto the mirror of the conventional DSLR.

However, over time, any dust falling on the mirror of the conventional DSLR will get thrown off into the inside of the camera, and some of the dust will ultimately settle on the sensor.

So in the long term, there is probably little difference in the dust susceptibility of both types of camera. More important is the question of whether the sensor has ultrasonic cleaning, since this is a great help in shifting dust off the sensor, whether mirrorless or DSLR.

Steinco Industrial Solutions, Inc.