In jogging you get more air, it's easier to respire etc. So you may find it easy in comparison. However on the treadmill you are presented with information on your running such as calories burnt, time etc. and lose your focus.
Treadmill does more harm i think on your joints because i get injured from walking on treadmill more than i do sprinting on the track.
Running on sidewalk is bad for your joints, I can tell you that. So if there's no street where it's safe to run or a track that's open to the public, a treadmill is a good alternative.
Plus it's just so hot outside, man! Like I went out to the garage for two minutes and I come in all sweaty and junk.