If you have an MBA in Marketing, I assume you're over 25 years old. Therefore, I am sure you are old enough to make decision for yourself. Nobody in this forum nor your relatives can force you to go for a Ph.D. The decision is in your whether you have motivation and diligence to complete a Ph.D. In brief, NOBODY CAN TELL YOU WHAT TO DO. Keep in mind that a college degree requires times and money. Time is Money...so, Ph.D = Money^2
Do you want to be a university professor? If so, do the MPhil/PhD. If not, go use that MBA to start making some money!
Write a marketing plan for yourself, for what you'd like to do. Would the MPhil or PhD let you offer major benefits to your target market that you can't offer now?
What country do you live in? I'm curious since you mentioned the MPhil degree. What business sector to you work in or aspire to work in? For example if you work in the energy industry you could earn a engineering technology degree which perhaps would help you understand some business issues.
