I think it depends on what kind of planning you want to do and the focus of the MPO with which you are working . Here in the Midwest, in my experience anyway, most of the MPO's exist mostly to administer Federal transportation dollars and do transportation planning in order to meet Federal requirements. However, there are many exceptions and some MPO's provide more traditional land use planning services, as well as economic development programs, environmental compliance, and GIS. If your interests go beyond transportation planning, make sure that the MPO will give you opportunities to work in several areas. Starting out at an MPO may give you a breadth of experience you might not get working for a municipality, so it could be a very good way to start your career. Good luck.