I think it's too early to tell, there are no statistical studies made demonstrating that someone with an MBA in project management has advantages over non-MBA accredited project managers.
However, an MBA is usually a huge advantage for anyone career wise, since MBA students are exposed to many fields other than project management, including marketing, business, finance, international trade, accounting, executive management, etc... Additionally, there are more and more companies asking for an MBA degree as a requirement for their top tier positions (including project/program management positions). Not to mention that an MBA holder will definitely be promoted faster than someone who doesn't have an MBA degree.
I think if you have the chance right now to do an MBA, then don't miss it. It can only do you some good to your career.
I think that an MBA holder is slightly overqualified for low level project management positions (such as junior project managers and assistant project managers).