Great question.

An MBA is more flexible in terms of careers than an urban planning.

That said, since you already know your career will fit whichever option you choose, choose the Urban Planning.  Why, pray tell?

MBAs at most schools are more costly than Urban Planning degrees.

Since you will achieve the same results financially, you will save money in taking the Urban Planning degree, without any opportunity cost in not selecting the MBA since your career has already been selected.