Hello Goswami

placement in both the colleges for both of these branches are around 80%-85% in IIT Bombay the average package for civil engineering was around 7-9 lakhs per annum and the lowest lies around 3-4 lakhs for IIT Dhanbad the average package for mechanical engineering is 7-8 lakhs per annum and the lowest lies around 3-5 lakhs per annum but i will recommend you to choose a branch which you want to do then only you will succeed in future and get a good placement

hope this helps you