So Is one easier that the other ? the big question.
scientific research has shown that running on the treadmill is roughly the same as running outside if you make a few simple adjustments.
for example , by setting the treadmill at a 1% grade, you can offset the lack of wind resistance and the belt moving under your feet to make your run the same effort as running outdoors.
Other benefits of treadmill running is the use of ankle and wrist weights without looking like a strangely dressed space man running down the street ! I also like to train without a shirt in summer as I sweat like race horse. Being indoors has its advantages.
You can also dramatically increase your cardio on a treadmill without concern of fatigue or " getting back home" meaning you can push your limits through the roof with the knowledge that you can simply jump off and recuperate for a minute or two. I find on the tarmac I keep a few reserves as to tackle a hill climb or grassy section.
But in saying all that It still as monotonous as hell running on the mouse wheel for km after km.
Where as running outside at 4:30 am when the sun comes up and the birds are chirping, wind in your hair, all that jazz, is revitalizing!
Also once I start my run outside I WILL finish it without cutting any corners or stopping as I might on a treadmill I get no interference or excuses, I have to get back home.