Ultimately you have interest in MBA then which ever field you choose does not matter much. As per the eligibility criteria for management course, after graduation in any stream you can take up the MBA program so be it any background. 

As per your options given, most of the students choose MBA after Btech and less after civil exams. The scope also after btech and MBA shall be much better than the other option.

You can get into areas like team leading, project managers and even in areas like recruitment, business development, and budgeting. Even the job options widen up than engineering which can limit you in your specific area of specialization.

Whichever path you choose, you should be aware of pros and cons for both the fields and later decide accordingly.

Good Luck.