You can control the weather, the treadmill, speed, incline and the only variable is your body. Treadmills can be more forgiving on the joints and it's better situation for controlled training.
But, there's something to be said for training in the elements.
Yesterday was a glorious near 80 degree day in Kansas City, and so much of a good day, I took off for the jr high track near my house for some sprint intervals after dinner. It's not the most ideal time, but with two small kids and mommy and daddy working full time jobs, AND the weather was FINALLY cooperating in Kansas City, you take the time to train when you get it.
I waited out dinner for around an hour and left the house after kids went to bed, but that still didn't ward off a little heartburn during the workout. Taco Tuesday is always better going down, and not up.
The treadmill is great for dialing in a speed and forcing your body to maintain that speed. With the outdoors and especially on a track, it's up to you to push your body to its limits. The track situation is great because they are usually level, so no inclines and no traffic intersections to deal with when you run on the sidewalks or streets. School tracks are also usually built from a composite material that's more akin to hard rubber, which lessons the impact on joints and avoids cracks and uneven surfaces that could be a tripping hazard.
I've also just ran to time in the past, while this session I ran at 200 yard intervals. 200 yards sprint, 200 yards recovery. Time wise is was around 40 seconds of sprinting to 2:10 of recovery. Pretty spot on.
With a 15 minute warm-up run to the track, I was off for a set of 8 before trotting back home.
As you can see, I ranged from 35 to 40 seconds of sprinting over 200 yards. It was a completely different experience than the indoor track or treadmill. By the 175 yard mark, my legs wanted to give out and my lungs were on fire. It was nice to push myself to the limits in a new setting.
What's sad was at the last sprint, I wanted to get a sub 37 seconds, and produced my second to worst interval. Go figure. I was toast.
Keep your eyes open for better weather and hit the track for some speed work. I highly recommend it!