Which should I prefer, civil engineering at IIT Hyderabad or ECE at ISM Dhanbad?

Irrespective of the IIT tag you go for, Civil Engineering always has limited placements. Civil Engineering has become an outdated branch with little or no scope left. Although there are people who go for various options after pursuing civil engineering, which may include UPSC Examinations. What I mean is that Civil Engineering is not the right choice if your goal is placements. In that case consider ECE from ISM Dhanbad. Owing to the increased research in Machine Learning, the ECE branch is expected to grow ten folds in the coming future. 

Additionally, ECE in IIT (ISM) has a pretty good coding and robotics culture, so you have both options, going for core jobs or CS jobs.

Ultimately, it’s all about personal inclinations. All I suggest is that never go for a mediocre rank just because a certain IIT is better than the other. The choice of your branch is equally important as the choice of your institute. Moreover, the Civil engineering scenario is similar in every IITs.