I used to run quite a lot until I had a leg surgery (which was unrelated). I always preferred running outdoors. Treadmills get very boring and repetitive.
Outdoor running is almost always a better workout if you sincerely try to run. Treadmills have a fixed pace which keeps you honest, but the hardness of the ground, the variation of the terrain, and the lack of handrails build up muscles in more of the body and increases balance and endurance more when running outdoors.
These days, more and more people keep a treadmill in their home or use one at the gym. Treadmills are easy to get on and go because they are right there. One nice thing about treadmills is that you are able to keep track of your heart rate, distance traveled, and the incline of the machine. On the other hand, running outside is much more enjoyable and feels like more of an achievement. And it's also can be softer on your legs which makes you less prone to injury.
For me outside running is the better choice, and here are some reasons why:
1. Better Overall Atmosphere
Running outside means fresh air, changing scenery, and just a more enjoyable experience than being on the treadmill. To add on to that, it seems like more of an accomplishment to run from point A to point B than to run on a machine. I'd take jogging outside over running in my house any day.
2. Better Workout and Experience
I just feel better after a long run outside. Of course, the workout is also more intense when you run outside. Sure, it might be a bit harder on my legs and back, but that doesn't deter me from choosing the outdoors.
3. Multi-Tasking
While I don't always do anything on my run other than running, I can if I need to. For instance, I can jog a few miles round-trip to pick up something small from the local Wal-Mart or grocery store. On the treadmill, I can't do anything but stare at my television set.
4. There Are Other People
Sure, I can hop on a treadmill at the gym and talk to the person next to me, but that isn't very exciting. Getting in a run outdoors with a friend or two is a great experience. Also, you can challenge each other to go farther and you have to keep up with the other person.
5. Easier to Keep Doing
Getting up and getting outdoors each day is easier in the long run than simply hopping on a treadmill. It's difficult to make a treadmill exciting for very long. Therefore, I'll always stick with running outside because I can change my routes anytime I want.
Running—whether on a treadmill or on the pavement—is an incredible way to stay in shape. In the end, it doesn't matter where or how you run as long as you do it somehow. No matter what, running in one form or another is rewarding. I think you'll start to see your leg muscles, calves and thighs especially, become more defined and muscular. Both treadmill and outdoor running are similar workouts for your body. Just get out and exercise regularly! Hope that helps!