I definitely prefer to run outside simply because it's prettier and more interesting. I get bored and discouraged when I run on treadmills (I've only gone that far already?!?! And I'm running that fucking slow?!?!). Of course, if there is snow I'll run inside (I love running in the rain) or if I want to get really specific distances in. Also where I live now is in some little mountains, so it's a bit hilly to run outside.

To each their own, I don't get why it's a big deal!