Physics wise, I agree with Mangetout and Alex_Dubinsky.
However, for long (40 mins +) steady state runs, I think running on a treadmill is more difficult than running outside on a clear day. I think running on 0 slope is near equivalent to running a bit downhill. So, I run on a 1 or 1.5 to simulate outside conditions, i.e. even on a clear day there will be slight wind, some uphills and downhills, etc. However, running inside on the treadmill, one must be consistent with the pace. Outside, I can deviate a bit: slow down a little bit into the wind and uphill, recover on the flats, and pour on some speed (even if I don’t personally realize it) when I’m going downhill or when I’m wind-aided. It’s with those factors that I find it easier to run outside and have better times overall, because inside even without those factors working against me, I’m still running on a slope upwards, and trying to meet that pace consistently gets very tiring as time draws on.