alt Hacker NewsStrength training has more of a positive effect on body composition.
The problem with doing a lot of cardio is that you need muscle to burn calories (especially so without injury and as you get older), and too much medium intensity cardio will start to chew up lean mass.
No harm in doing a bit of both though, especially if your goal is fitness/maintenance rather than maximum strength or a particular look.
I think it's a bit of an exaggeration. There's an interference effect but there's people who compete in natural bodybuilding that run marathons as well.