There really isn't a definitive answer to this. Every module has its own advantages and drawbacks both in their practice in the business and the benefit to your career. And honestly, what module you're specialized in should be the least of your concerns.
Put continuous, effective learning first -- and the money and promotions will follow in some form later on. What you learn from one module and technology will help you become ready to take on another. Due to the integrated nature of the SAP modules, it is useful to have exposure to several modules.
Your end goal shouldn't be to become certified or specialized in one module or another. Instead, your ultimate goal should be to focus on finding a team of people with whom you'll enjoy working with and will fully support you in developing your career.