I only run on treadmills. with tv or at least a comic book in front of me. Running outside is so frigging boring I simply could not gather strength to do it.
There may be at least one good reason to prefer running outdoors to indoors, though. Maybe I've mentioned it here before (even in this thread?), but running/walking outside jogs your brain (hippocampus) because unlike on treadmill, you'll need to use your spatial awareness. Good reason also to switch off GPS while driving, unless you desperately need it. http://www.msnbc.msn.com/id/40138522/ns/health-mental_health/t/gps-addict-it-may-be-eroding-your-brain
I saw also another study where they were comparing people who train indoors (lifting weights in a gym) to people who walk outside, and found the same "shrinking hippocampus" with the people training only indoors, but they didn't make the link to spatial awareness. In fact, they concluded (IMHO wrong) that the reason for this is aerobic (walking outside) vs anaerobic (gym) exercises, but in order to make such a conclusion they should have had a third control group doing aerobic exercises indoors.
I personally prefer e.g. swimming (indoors) though, especially now when it has been rather cold in here. I swim 2km every Saturday, on top of MA exercises that's pretty much the aerobic exercise I get (and need, frankly). I guess my hippocampus is shrinking fast as well, not much spatial awareness needed in the pool. Maybe I should take a second job as a taxi driver.