In the realm of AdWords, metrics are all important. They are what you judge your success by, so knowing which one is the more accurate representation of your success is essential. Deciding upon which metrics to take account of depends in large part on your priorities.
What is perhaps the most important metric in the minds of many AdWords operators is the CPC, the cost per click. This tells you about the specific cost each click for an advert entails. It is very convenient thanks to the ease at which it can be calculated, and because people can draw more similarities between that and the amount they're bidding on each click.
The other one that many people use is the CPA, or cost per action. This metric tells you more about how well people respond to your website, rather than your advert. It is quite difficult to calculate, but it can give you a lot more information about whether or not your clicks are actually achieving anything substantive.
The answer to the question of which one you should use is both. Many people rely solely upon the CPC to tell them how well they're doing, but this tells them very little about how much these click-throughs are actually worth. Likewise with the CPA, it tells you very little about how the actual advert being displayed is performing. Using both can give you the information you need to refine your advert for maximum click-throughs and refine your website to achieve the maximum number of conversions.