1. MBA is for management skills irrespective of the specialization you do.
2. MBA will give you more insight about the way business functions - purchase,sales, marketing, HR etc.
3. BI/BW is related to the database, it's attributes and in turn the report generation.
4. With the new dimentional module like SAP CRM you can opt for Sales and Marketing, if it is SCM then purchase, if it is HCM then it is HR. Else, you can also opt for Systems as a specialization for your MBA.
5. In short, with your BI/BW knowledge, you can opt for any specialization which interest you both. Do not always look from market perspective, your interest should also consider for the specialization subject.
Hope this will be useful
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MBA covers general Management skills which are essential for any B School passout.
HR : It covers stuff more about people management, Recruitment, etc. It is not much mathematical.
Marketing: It is very important component of business. It deals with not only sales but with strategic Business decisions. It typically needs data cruching and analytics for decision making. It needs to process sales data, consumer preference data, etc.
Finance: It involves numbers and excel based financial modelling. It calls for deriving business decisions with numbers to support. It deals with investment decisions, accounting, Profitability analysis, etc. It calls for heavy number crunching and use of excel like analytical tools.
Ops/System: It deals with project management and production efficiencies.
This is just insight into MBA specializations, but it is upto your interest and future aspirations
MBA will give you varieties of management knowledge with helicopter view on them. That will enabled you to understand interconnection in between actions and strategies as well as reasoning why a decision was made. At the same time, as normally master degree will have to do, sharpening their analytical skills in searching solutions and managing challenges. A person with BI/BW experience like you, will then understand the importance of KPI and how typical reports that you made can be useful and critical for management in making decision. This will help you to enhancing analytical skills and broadening your knowledge. Keep in mind that MBA is very general and not depending on technical skills. Many of technical background people normally did MBA to learn general knowledge and at the end of the day it depends on where you want to drive it next. Nowadays, there are many MBA with specializations like in Finance, International Business, Technology management, etc - depending on country and universities. The best is to see what subjects they are learning and detail syllabus of those subjects. And inquire to respective academics on detail of subjects before you decide which one. I hope this is helping you in direction. Good luck!