General rule of thumb for running on a treadmill, taking most all variables into considertion, is that a 1% to 2% incline on the treadmill is required to equal running on a level road course.

I like running on both. A treadmill CAN have less impact on joints, allow for speed and grade work, and allow you to stay close to home (Kids). Of course, nothing can prepare you more than getting out and doing real work on a real road. I tend to run for time and intervals on a treadmill while I go distance and pace on the road. Of course, either will be of benifit and just getting out and moving is what's important.

Regards.