I think it would be generally difficult to put running vs cycling head to head without taking into consideration:
1. Whether or not we are talking about someone who is off the couch with no fitness history, someone off the couch with fitness history, or someone who is already in good shape
2. We talking steady state and moderate or intervals, hill repeats etc?
3. A person who needs to get stronger in the legs or someone who could benefit more from losing weight/getting smaller in general.
Cycling would probably generally be better for a person who is simultaneously lifting weights and trying to build some muscle. Running would probably generally be better for a person really trying to cut some weight. As I understand, lots of running under your own body weight will send a message to your body that it needs to get smaller and cut non essential weight (including muscle) to the task of running. Extreme example: This is why a lot of long distance runners look like broken down concentration camp victims. On a bike, much of your body weight is distributed through the mechanics of the bike itself propelled by your legs. It would seem to follow that the message to your body is grow stronger legs. Extreme example: This is why many high end cyclists look like sticks on a giant set of legs. Of course, both of those examples are dependent on volume, intensity and background, which goes back to the original question of what kind of running and biking exercise are we talking here, who is doing them and what kind of background do they have?
All that may even be a moot point because, no matter what, runners are going to tell you to run and cyclists are going to tell you to cycle.
-best just to experiment and see what works better for you.