Hi.

You have already chosen your post bachelor degree plans i.e. clearing govt. job exams but the question remains that in which field are you willing to get a government job. If you want to know which of them (Civil or Mech.) has more chances of getting a government job then the answer is actually Both of them. Because both of them are separate fields of expertise and students of one civil will not contest for mechanical jobs and vice versa.

Good Luck