Both IIIT Allahabad and MNNIT Allahabad are excellent institutes, providing sufficient placements in the circuital branches of engineering. But, IIIT Allahabad has slight advantages over the other, considering certain parameters in branches, which are discussed below in the following points.
For M.Tech in CSE or IT, IIIT Allahabad is a better choice, provided the coding culture and placements in those branches at IIIT.