If forced to choose inside, I go for the treadmill.  18 laps/mile is more mind numbing than the treadmill.  At least you can watch TV or something.

If I had control over the weather and where I can run (meaning, not in the town I live in presently), I would choose running outside. I run outside in the summer, near the ocean, and I love it. But I cannot have my way with the weather and the location, so I choose the TM. There is no track available near me and I don't want to drive to one. Also, I love the solitude on my TM. You guys really do not understand the peace it brings me. I can hear the sound of my footsteps and feel my heart beat and my lungs expand in the same way as all of you running on the road. In my mind, I can see whatever road I have seen in my travels. It is a very internal thing for me. Just like running on a quiet country road on a winter or summer night must be for you, I'm sure.

EDIT: I spelled "hear", "here". I had to correct myself. My brain is weak and mushy. grrr...

Oh, I just read Oski's second post. You want advice, not an essay on why I love the TM, I get it.  I say go with the TM. Softer surface is gentler on your legs and joints. And you don't have to worry about other people on the track.

For me, this depends on a number of factors:

•  If there is snow on the roads in my neighborhood, I try to avoid anything more than a short, easy run because my delicate calves/ankles do not like running through snow and I'm afraid of getting injured.
• Doing too many runs at the indoor track seems to start to bother my delicate left ankle, so I try to switch it up by running outside when I can
• Running more than an hour on the treadmill seems to hurt my delicate knees/ankles plus it makes me want to stab my eyes out.  I won't consider the treadmill as an option for any run longer than 6 miles and is really a last resort (indoor track is closed and running outside is impossible and even then I'd be tempted to skip a run instead).
• If I need to hit a certain pace for the run and outdoor conditions make that unlikely or impossible, I will choose the indoor option knowing it's what I need to do to successfully complete the workout.
• What option sounds the most pleasant (or the least terrible).  If it's -10 out with the wind chill and dark and the indoor track sounds appealing, I will choose that option

Usually I prefer outside, but I've been a bit of a wuss this winter and have been probably doing half of my runs at the indoor track.  Partly because of cold/wind and party because we've been getting a small amount of snow every single fucking day and as a result the roads in my neighborhood are never clear enough to run without a fair amount of slipping around.  However, my indoor track is 3.5 laps / mile. I can't imagine running 18 laps per mile.  Not sure I could do it mentally and pretty sure my ankle wouldn't tolerate much.

I'd love to have access to an indoor track.  As it is, it's the treadmill for me.

Cold doesn't bother me, but running on snow and slippery roads is useless when you are trying to hit specific paces and do specific workouts.  I am trying to teach my legs to run faster.  Slow runs in the snow won't help with that.

Track. But just because I refuse to use the treadmill.

I'd choose the treadmill if I HAD to run indoors, but not because I find it less boring; an indoor track would slaughter my ITB, chew it up and spit it out so that it could do the same thing all over again.