I actually find it 100% easier to run outside during winter compared to summer. You don't feel the cold in winter after the first couple of minutes although I do have to wear a headband to keep my ears warm. I planned to run 10km this morning and would have been on to smash my personal best time if I hadn't given up at 8.5km because it was so damn hot. I don't think I've sweated so much in my entire life. On the plus side I managed to run 8.51km in just under 59 minutes which beat my previous best hour distance of running 8.16km. In hindsight I should have run for another minute to get the record on Endomondo.
Running on a treadmill will be hot if you don't have aircon.