My own personal experiences, running outside has better influence not just on your knees but your lungs. To start off with, as you run on the treadmill and increase intensity (via speed, incline, and ect). It applys more pressure on your knees that cause long term pain and irritation to the joint.
Now with the cold air running through your lungs. Running when the sun just is rising, seeing your breath fading away in the cold breeze air. Actually makes you lungs become stronger and healthier. Cold air makes heats and humidify the air before hitting the strains in your lungs. So, the next time you are running and feel that burning sensation after you take a five second rest. Is actually your lungs building stronger and more conditioned.