Assuming that you have tried multiple times to start your car with both keys and it is consistently one key that works and another that does not, you can affirmatively say that there is a difference between the two keys. (Not rocket science. :) )
As user22838 said, replacing the key could fix the problem. That being said, however, a physical inspection of the key could reveal the problem and a possible workaround solution. Is the key bent? Is it worn? Compare the two keys for physical differences.
If the key is bent, a very careful pressure applied with a soft material (ie., wood, not metal) could bring it back straight and put it in functioning order.
You could also check out this article for more information and tips.
