It all depends on your previous experience and knowledge. Start by taking a look at the "Skills Measured" for each exam:
http://www.microsoft.com/learning/en/us/exam.aspx?ID=70-502&locale=en-us#tab2
http://www.microsoft.com/learning/en/us/exam.aspx?ID=70-503&locale=en-us#tab2
Observe each of the entries in either list and then think about how close they are to things that you have done before, and therefore how easy it will be for you to learn them. If you have a lot of experience writing WinForms applications and/or manipulationg
the html DOM with JavaScript, then the WPF topics will be realtively easy for you. On the other hand, if you have done a lot of work with WebServices and Windows Services, this will probably help you understand WCF.
Besides considering the difficulty of the exam, think about which of these subjects will be more useful for you in the future, which depends on which kind of applications you expect to be developing. If you are going to spend a substantial amount of effort
into learning a new technology, it ought to be something that you will actually find useful, not just something that is easy to certify on.
