Before you begin…. What are your AdWords goals?
What do you want to see happen through AdWords? Do you want to get X number of clients per week, sell this particular product X number of times more than you have been?
Are you trying to get leads or are you trying to get people to download your brochure? Figure out your goals and work towards meeting those goals.
Having goals in mind before you get started allows you the ability to review your accounts performance and determine whether your return on investment meets your expectations.
While the CPC in account (a) may be higher then another, if the conversion rate is better in (a) then you may deem that more successful. On the other hand, you may also want to look at specific conversion date. Different Conversions (desired actions such as form fills, quote requests, ecommerce sales, etc) have different values. Make sure you are tracking different conversions separately so you can track as efficiently as possible.