And it is of course, but what's more important knowing whether God exists or the age of the Earth?
Why do you ask that as if one must choose one or the other?
The latter, obviously. It's interesting, and a fundamental datum in geology, astronomy, biology and other sciences. The theological question, OTOH, is really rather boring, will most likely never be resolved, is of interest to psychologsts and neurologists and so forth, but has no practical value. Understanding where believers are coming from may help in the War on Terror, but that's not the same question.