I believe it's about how much money you are willing to spend and the quality that you are looking for. I never owned an elliptical trainer but I did purchase my treadmill several years ago from Sam's Club and it has held up well. Now I am looking at more expensive machines that have more features, better impact absorbtion, sturdier, ...
I also bought a stairclimber form walmart many years ago and it sucked. Of course it only cost $100 so what should I expected. One good thing about Walmart is that they will take back almost anything. Buy one that you think you will like and if it turns out that it's not what you want, take it back. Just remember not to throw away the box it comes in until you decide whether or not it's a keeper.