When I was looking for a treadmill about 5 years ago, one brand that I saw was Nordictrac. After seeing that brand of treadmill, I thought about the ski machine and I eventually got one instead. I find several advantages to the ski machine:
1) much lighter in weight. A treadmill is 250-300 pounds. Nordictrac is about 40 pounds.
2) it makes less noise, especially to a person living underneath. a treadmill makes a loud pounding sound to those underneath you.
3) less impact on the knees.
4) price – I got a used Nordictrac in good condition on craigslist for $40. Used treadmills go for more, and a good new treadmill can be upwards of $500 (I got one about ten years ago). And then you have to schlep it (see item 1 above).