There are obviously many different reasons to pursue one or the other, but achieving both is now very realistic and affordable. The University of Phoenix, for example, offers an MBA program which includes PMP certification as an option (you still need to meet the work experience component of the PMP Exam). Many colleges and universities are offering MBAs with a Project Management concentration. While they don't necessarily include the PMP certification option, you will be well equipped to pursue it later.
Besides affordability and convenience, another reason for pursuing both is they can offer you a different perspective on similar topics. For example, risk management in my MBA program looked at issues surrounding investment portfolios and business decisions. Risk management as a Project Management Professional takes a broader view of project related issues for any industry. My MBA program taught me how to construct budgets -- my PMP training taught me how to manage budgets.
My suggestion would be to pursue both if possible.