With the kind of specializations coming up as a part of MBA and the opportunities that open up after each of those make it a bit confusing for the students to select their specialization. Exactly what you are going through right now!
Nevertheless, in order to resolve this one needs to get into R&D about each of the streams. You can start off by reading as much as you can on each of them. Log on to http://www.youngbuzz.com/careers/career-options and check the section for management. Secondly, you can also talk to various professional from the fields who can give you the first hand information about the industry and lastly talking to your professors.
While you do all of this also pay attention to what kind of personality traits are required by the streams like HR,Marketing etc. Get into introspection about yourself too, to understand what kind of person you are and what are your dominant work values. For example; marketing professionals are required to be very driven, social, high on convincing and influencing abilities, while those from HR are required to be solution-oriented, good at interpersonal skills, excellent at decision making and team building etc.
Once you are able to do this you will be more-or-less able to decide a stream for you.
Hope this helps.