In a study that used lab rats running treadmills to judge the effects of human cardiovascular exercise, researchers looked at the different benefits offered by interval training with peak intensity reaching 85% of capacity compared to a more moderate 60% pace of steady state running. Findings were reported in the Journal of Applied Physiology.
Although high-intensity interval training improved functional capacity faster than moderate intensity running, muscle adaptations were pretty much the same between groups. So if you want to increase your level of conditioning, go with HIIT. And when you want to give yourself a break from intense training, jogging the treadmill isn't likely to hold back your long-term progress.