i always preferred actually going outside to exercise rather than using a treadmill. makes the workout go by faster
Even though there's nothing better than fresh air and a change of scenery to make exercise more fun, moving is moving, wherever you're doing it; it's much better than sitting in front to the computer for hours like I'm doing now... :-)
I have a treadmill and although I don't use it like I should, neither do I go outside and walk like I should either. I do have my treadmill in front of a window so I can watch the birds and whatever else might be lurking outside my window and can easily spend half an hour or more on the treadmill, with my earbuds in, listening to my tunes. The problem is the motivation to get in the room and get started; that's the same problem I have getting outside (along with the almost 100° temps we have). Once I get started on my treadmill, I'm fine, but I can't walk very long/far in the heat.
I don't think so that treadmill is as good as walking outside because when I go for a walk out, I don't count the time that how much I walked. I think that is best for you, and I don't get tired quickly with this, and this is a perfect thing for everyone to stay fit and healthy.
I have a friend who lost a lot of weight on her treadmill. She was very devout, and would walk first thing in the morning and say her prayers and think over her day. She even wore out the belt and had to have it replaced! And she lost a LOT. She also lived in a forested area with curvy roads that were 'basic rule' (country roads with no long views, and people can drive 55 mph on them) so there wasn't much safety from inattentive drivers if she went out running. (The issue of her being a woman alone running on country roads probably also had an impact on her decision to use a treadmill instead.) If you think you'll use a treadmill, especially if you don't think you are as likely to run outdoors than you would be on a treadmill, go for it.
I always walked/run outside. I find it more relaxing. I did buy a treadmill for winter and when days were crappy, but honestly I hate using it. For me, 5 minutes on the treadmill feels like an hour and I'm constantly checking to see when my run will be over. I also don't like how it feels to run on the treadmill. Now, I don't have a fancy treadmill that does inclines, but it just feels unnatural to run on.
For exercise, they can be better -- if you have a high tech one or go to the gym you can build in hills. You can alternate jogging with walking with hills if you want to do interval training. The surface is supposedly easier on the feet than pavement. You can watch TV while you do it, if that's your thing. You can vary the speed. For pleasure while you're exercising, there's no beating getting outdoors, though, especially if you can find good long walks that have hills and dirt trails. If you exercise in the early daytime, you'll get your Vitamin D. Can't get that in the gym. Then there's the weather -- if you're not tough, you'll skip it a lot, whereas you won't have that excuse to skip a treadmill indoors. I've had to stop walking due to injuries all over, and I would have preferred to keep running or bicycling if injuries hadn't stopped me, but I always prefer getting outdoors and no weather ever stopped me. Ran or walked in rain, snow, and even ice. If you're near woods, you can see animals if you go at night and in the winter you'll have the animal life and the whole area to yourself if you go at night. Just depends on what you're going for, pleasure or the best workout, because the best workout might be the treadmill. One other advantage to walking outdoors -- you can walk as long as you like. If you're using a gym, they frown on someone hogging a machine for a couple of hours or more. So also depends on how long you go for.