Should I choose mechanical at Thapar University or metallurgy at NIT, Jamshedpur and Durgapur?

I can only provide you with some basic facts about Thapar University. As some of my friends are from NITs, I can assure you that NIT Jamshedpur is definitely a good choice for Metallurgy. 

• It is an institution of national importance under the MHRD.
• The faculty members are good there.
• As far as placements are concerned, Tata Steel, Vedanta, Hindalco, etc. are some of the top recruiting companies.
• You can also sit for placements in IT firms and organizations.
• Metallurgy is one of the oldest branches all over India. It has great alumni support. 
• 'Technica' is the oldest branch fest of NIT Jamshedpur.
• The new AC furnished labs on the campus are well maintained.
• Jamshedpur has enough industrial and research exposure through TATA Steel and National Metallurgical Laboratory.

Being one of the oldest and most reputed branches in NIT Jamshedpur, It is suggested that you go for Metallurgy. There is a huge scope in Jamshedpur as well as all over India.