A treadmill is better on your knees as opposed to road running (tar/concrete) because the road will not "give in" when you land and a treadmill will TO AN EXTENT. Ideally, I would look to run outdoors when possible on a football, soccer field or any large grass surface that is mowed and manicured. As far as running and biking, they both give you cardio and the bike is easier on your knees. I would look at an eliptical machine. I use that daily in college and like the results. It also is more of a full body workout but doesnt leave me to tired to lift hard as well. If you live somewhere where you can not run outside, I would do my sprints inside and maybe 2 days of distance running on a treadmill and then would alternate with the eliptical.Although sprinting can also be done on a treadmill if you wish. That should give you good results and not wear your body down to much. Here is a basic sprinting routine we use in the early fall.
10X 8sec. sprints (100% effort outdoors or 16mph on treadmill)
8X 16sec. sprints (75% effort or 13mph on treadmill)
hope this helps.