I am a runner as well. My
personal preference is
running outside. Junk the treadmill!
Running outside provided too many benefits that a treadmill does not. First of all, you can have nice scenery outside, fresh air, and get some vitamin E from the sun while you're at it. Also, it's too easy to step off a treadmill than finish a run.
As far as interval training, you can mix up your running. Some days have nice short runs, some days work on speed, some days work on distance. It all depends on what your goals are, especially where running is concerned.