Which universities are you considering now? What changed your mind?
In my opinion, an MBA is valuable as a post-experience qualification. The syllabus is more general management, so I reckon it's most useful if you're at the point of transition from middle to senior management (or if you want a career change, e.g. to move from industry into consulting). If you do an MBA very early in your career, you will have learnt a lot about general management, but without the experience to move into a general management role. It might still be helpful and the knowledge may serve you well when you get to senior management.
Obviously, an MSc in Marketing will be useful for marketing roles. In your early career, you are going to be in a functional role. This is important experience. Functional experience is your career foundation. Lack of this experience is why some management consultants can struggle to move from consulting into industry.
Personally, I would rate an MBA and an MSc equally. Either would be a useful qualification. At least in the UK, however, you want your CV to show that you have great experience in business. More than one Masters qualification looks odd to me. You want to have the right balance between academic qualifications and experience. That balance should be biased towards experience. When hiring, I'm really interested in what you've achieved at work, not what you've studied.
If you're still considering an MSc in Marketing, then my advice would be to find out in detail about each school's track record with placing students in work. The economy isn't good and it's tough to find work. So you want a school that is genuinely excellent at helping its students find an appropriate job.
If you want to work in the UK, you should check if the MSc exempts you from the Chartered Institute of Marketing exams.
It would be great if you could share your work trajectory in the field of Mktg and then the switch to General Mgmt.
Since you asked, I left university after my batchelor's degree. I was very lucky to get a place on a 'fast track' graduate entry programme to do marketing with a well-known multinational. The company sponsored me through a business school executive education programme part-time. I did a number of years in consumer marketing, global product development, etc. Then I was lucky to land a role in an internal consulting/strategy group in industry, eventually becoming its corporate director. I left to become the CXO of another organistion. I never paused to do an MBA, which might have slowed down my career. (I made CXO level in my late thirties.) However, I'm now returning to business school for a senior executive programme.