Ch'i

Depending on your physical condition, of course:

I run every day, and have used a tredmill quite a few times. I've noticed a definite difference between the two in favor of not using one. When you're running outside you can wind-sprint, and develop better stabilizing muscles from running on uneven ground. Though that can be negligible. I prefer running in a forest because of the changes in pace, trail inclination, direction, and the fresh air for instance. Any outdoor running beats a tredmill. Also, tredmills are the single greatest waste in money I've ever seen. Why spend $600-$3000 on one when you can get a great pair of running shoes for $70?

Goodluck. Run the track.