I wrote last year about
the differences between a track and a treadmill at my site.
In brief:
Treadmills are softer and cushion your impact more than running outside, which leads to less of an impact. Less of an impact leads to less bone growth.
You don't have to break the wind on a treadmill (no lewd jokes, please!) where you do when you are outside.
You generally don't have to spend as much time thinking about how to keep yourself from getting hurt outside quite as much as you do on a treadmill.
You also want to be sure to use a 1-2% grade on a treadmill to avoid hyperextending your back and injuring yourself, which is much more difficult to do outside.
Despite all of that, treadmills are tougher than running on a track. Treadmills are boring; I rarely do more than a mile or so at a time on one and it is torture any time that I do. I'd rather go outside no matter what the weather.
