Treadmill was definitely 'easier' for me, but utterly boring. When I started out running, I started on the treadmill, I was 2 stone overweight and didn't feel confident running outside with all my wobbly bits

When I did my first run outside it was incredibly difficult, and I questioned whether I could keep it up! But a few weeks later I was fine and it didn't feel such a struggle. I still sometimes use the treadmill, but only to do intervals. If I
really have to, I also use it during the winter when it's icy outside.