Neither is a big deal for a career in public accounting. If you plan to work in corporate finance, an MBA has some value if it's from a prestigious school. Otherwise, don't bother with a graduate degree from a minor college. Instead, focus on courses you need for CPA licensing. And look over the free online sample CPA exam questions at CPA Exam Review, CPA Exam Courses and CPA Review Books - FFA to find out what you need to learn for passing the test sections.
I agree - if you want to work in the private sector an MBA from a notable school will help - but if you want to qualify for the CPA than I would do a MAC then you could teach a few classes if you wanted to.
It depends what you mean by "the accounting field" - corporate finance and treasury may well be more accessible with an MBA, but otherwise I always saw an MBA as a ticket to front-office management or strategy advisory.