I have personally obtained the best results by varying the cardio. I mix low intensity and high intensity cardiovascular exercise. I think variation is important to stimulate the heart as much as possible.

What I do personally is if the weather is bad I get on a treadmill and walk at a speed of 6 km/h for 5 minutes then increase it to 10 km/h for 5 minutes, then up to 11 km/h for a further 5 minutes and then finally I reach a speed of 12 km/h for 5 minutes. By the end I am sweating like as if I got out of the shower. I feel great and due to this I have been able to train harder and longer in the gym; my sleep has improved and I can eat more, because I am burning more calories.