There are a bunch of advantages treadmills have over the open road. You touched on the biggest one, convenience! You will be more likely to stick with a running program if you don't have to go out into the bad weather! Another advantage is that, depending on the cushion of the treadmill, it is less damaging on your knees. In time, the hard pavement of the open road will place ware and tear on your joints. Still another Advantage is that with a treadmill you can keep track of your speed, distance gone, intensity level, calories burned, etc. So I think a treadmill is a wise investment to make!
There's advantages and disadvantages on both sides of the table when comparing mechanical treadmills to manual ones. From a financial stand point, mechanical treads are more expensive to buy, run, and maintain. Even though allot of mechanical and manual treadmills are space savers where you can fold them up and depending on the brand of treadmill you get, mechanical ones still take up more space. Also mechanical treads have a constant belt speed, which can be an advantage or a disadvantage. I would have to say though that a person burns more calories on a manual treadmill because they're forced to physically move the tread opposed to a mechanical one where the machine does it.
So the bottom line is that you have a number of things to consider:
1. what type of treadmill can you aford?
2. How much space do you have for a treadmill?
3. What kind do you prefer, mechanical or manual because they both offer excellent workouts but you will be more likely to stick with a workout that you enjoy doing!
------------------
If you build it they will come
