I asked that question a few days ago and here's response 1:
It really what you prefer. Running is running running outside is great, I love it. It feels more open and there's more to look at. But running on a treadmill lets you run a little easier when its storming or its 100 degrees out. Lots of people are doing both. Switching from on to the other can be difficult but doable.
Best of luck to you!