BTW I often get asked my developers if I recommend taking the ZCE, Symfony2 or other exams. In general it depends on where you want to work imho and how you want to proof your qualification. Personally when I have to make hiring decisions I will always value open source contributions higher than any exam. But of course getting into open source can feel more daunting and less easy to "plan". However I think in many ways open source involvement will feel much more rewarding. That being said, from what I have been hearing (I took the original ZCE and the beta MySQL 5 developer exams) is that exams are getting better at actually testing to ensure that developers really know the given language/technology and less focus on obsure trick questions. So I have been hearing a few stories from people that told me that they learned important stuff while studying for the given exams. In that way exams might be better at ensuring that developers know the important things that go beyond the basics. Instead spending time on open source will teach you more about the practical aspects of development and researching. At the same time I guess that companies with a human resource department will tend to prefer people with certifications over open source involvement.
TL;DR: one can consider taking exams and doing open source development can be complimentary. if you dont have the time to do both, prefer certifications if you want to apply at a larger company with HR departments and open source at smaller shops.