Is it a fruitful decision to leave NIT Jamshedpur CSE for MNNIT Allahabad mechanical?

Well, it depends upon several factors. A few of them are as mentioned below.

• Where you possibly imagine yourself in the next 10 years and more?
• What has been the real motivation behind your aim for pursuing engineering? 
• What is the time period you are willing to assign for reaching your goal?

There are other factors, or possible scenarios, such as,

• Cushy CTC: In this case, CSE will be better than Mechanical Engineering.
• Higher studies abroad: CSE and Mech both will offer similar opportunities. 
• Competitive examination: In case you would want to take the UPSC examination, IES, PSU, or several others, choosing Mechanical will be better as compared to CSE.
• MBA from IIM: With regard to that, being a student from the mechanical department can give you an edge. As it will offer you diversity points. If however, your scores are low, then you will lose the edge.
• Work destination: If you want to work in Gulf in the future, then choosing Mech is better. If you dream of working in the US, CSE offers better options. 
• If you have a natural affinity towards automobiles then go for Mechanical Engineering. 

The above scenarios can give you possible average outcomes, but there can be vague contradictions as well. You also need to consider which subject interests you. 

According to various student reviews, the preference order for CSE and Mechanical is MNNIT > NITJ. You should always give preference to the branch rather than the institute. Hope this clarifies most of your doubts.