Was happy doing anything from 10 minutes to 90 on a treadmill but that changed when I lived to Dubai and trained for a marathon, almost solely on a treadmill. I tried running outdoors for a few weeks, but had to get up before 4am and even then it was too hot and humid by 5-5:30 to really get going (it was summer, so 30-35C by early morning, 50C by midday). Got badly sunburnt at 7am, had a litre and a half of water last me about 40 minutes - it was horrible. So went indoors and trained on a treadmill. Did two hour + runs and they were the most monotonous runs I've ever done. Just can't do longer than 30 minutes on them now, even with TVs to stare at!
To be honest, even without that, I'd still prefer that feeling of being out, especially early morning, wind, rain or shine, with very few other people about. There's something strangely thrilling about running through empty places that are normally busy, like a town centre.
