The following is just my opinion.....

To increase muscle and cardiovascular endurance at early stage I would say running on treadmill is more advantageous because it's easier to evaluate and control all factors relating to the training eg. speed, slope, time and also frequency of the training. Treadmill running could also minimize risks of getting injuries such as taking too much ground force reactions, getting shin pain from running up/down too fast over the slope, etc.

However, the athlete should be put on treadmill and natural running at some stage of the training. The athlete will be more confident and more familiar with the real venue.

I reckon that an effective training should have an appropriate combination of treadmill and natural training. Running on treadmill and naturally outdoor should not be compared, it's depending on what advantages you are looking for. Which is better ?...it's up to your purpose.