I very highly recommend Martin Gorst, Measuring Eternity: The Search for the Beginning of Time (Broadway Books, 2002). As a writer and documentary filmmaker, Gorst vividly illustrates in this captivating and character-driven narrative the fascinating, centuries-long journey by religious figures, philosophers, astronomers, geologists, physicists, and mathematicians to discover the answer to a fundamental question at the intersection of science and religion: When did the universe begin?—a question that arose from pondering the age of the earth. He very helpfully traces the historical process of these questions being explored and in such a way that it makes a good deal of sense how we landed on these huge numbers. (I was particularly struck by Georges-Louis Leclerc, Comte de Buffon, whose fascinating experiments led him to calculate that the earth must be more than an order of magnitude older than 6,000 years.)
Since those are not mutually exclusive, why not both?