Should I join TCS or IIIT Allahabad?

You should first ask yourself, “What do you want?” Do not go for M.Tech from IIIT Allahabad just for the sake of an extra degree. Do not make this attempt if you do not have that zeal to learn. If you want to have a good career option and you are ready to learn more and dive into the core concepts of your graduation then only opt for M.Tech. Pursuing M.Tech from a reputed college like IIIT-A will definitely help you. But if you want just a degree then you will be dragging a load of studies for 2 complete years which will not benefit you anyway. 

Job in TCS will always be an open option for you whether you pursue M.Tech or not. But if you have decided to pursue M.Tech, then the next question arises which college you should prefer to pursue M.Tech?

IIIT Allahabad is an institute with excellent faculties and a good algorithmic coding culture which is quite popular on the campus. This can really help you during the interviews with product-based companies. From a financial point of view also, it will take you quite a long time in TCS to reach the pay. But if you pursue M.Tech, you will land into a product-based company. So, think about it, decide what is best for you and go for it.