It's a well known fact that running is one of the best ways to get your heart rate up and exercise your entire cardiovascular system. But does it matter where you do it?
The answer to that depends on whether you're running to train or simply for fitness.
Treadmill running is easier in some respects because the belt assists with the turnover of your feet. This enables you to run a lot faster than running outside. As such, running on a treadmill may give you a slightly inaccurate report of your running speed. Because the treadmill also gives a little more than the hard surface of the road, running this way doesn't give you some of the muscle conditioning that you get from training outdoors.
Running on a treadmill is an excellent way to work on your speed. You're running in a controlled environment when using a treadmill so use it to your advantage by increasing speed for short intervals and slowing for recovery periods.
If your goal is to train for an event like a marathon, it's recommended that you do your training runs outside. Not only do you get used to running in different weather to prepare you for whatever the marathon day has in store, but the hard impact of the road or pavement helps to condition your soft tissue.
Incorporate a combination of both into your regime if you can, as this will help you build your conditioning and speed together as well as improving your endurance.