Running vs stair-stepper/eliptical?

When the weather is nice, I run outside on a regular basis, but since it's been cold up here in Wisconsin the past few months, I have to do my workouts indoors. I don't like using treadmills, so I workout on the stairstepper. I set the machine to a vigorous level, go for 30 minutes, at least 4 miles, and burn at least 400 cals each workout. I was wondering if it is easier to do 4 miles in 30 minutes on a stair-stepper or running outside. I just want to know if when I start running again, I will actually be faster, or if I will just run like I did before.