Here's what I would do. Go for the MBA. Here's why. As an entry level employee, gaining a background in the general principles of accounting, management, economics and how business operates will give you enough background to a.) be more valuable to your employer, and b.) decide if you want FURTHER education or certifications in other areas.
Think if it like this: You can get an MIS, but it won't really be until AFTER your degree that you start to think about whether you want to move toward project management, security, programming, infrastructure...but having a little of everything from your MIS will help you understand the bigger picture and help you determine your next steps in personal development.