Without any second thoughts, IITKGP is a far better option than IIT Dhanbad in terms of both worth and recognition. The primary reason is its legacy and early establishment. To give you a better picture I am going to entail the pros and causes of IIT KGP bringing to light both sides of the coin.

Pros

• You will get the chance to pursue the subject that interests you and fuel your passion. 
• Mechanical branch is much more preferred than metallurgy.
• IIT ISM is developing at a fast pace and will soon overtake other institutes after its inauguration of the biggest library in India.

Cons:

• Petroleum engineering does not guarantee good placements. Around 50% of the batch remains and placed and the situation is a result of a steady decline in petroleum industries in India.
• Dhanbad is not a very promising city in terms of exposure and opportunities.
• Electrical engineering failed to offer decent placement in the past, and the IIT tag does not seem to improve the situation.

The final decision is for you to make.