I believe MNNIT Allahabad being a NIT has a more student friendly and structured curriculum in ECE with quite a number of Electives like Economics and Humanities and Social Sciences as well as Management Subjects which indirectly might help you in your quest in UPSC. Having a larger batch with almost 10 Departments, you might find a good group including UG, PG and Ph.D students preparing for UPSC, which perhaps might just help you.

IIIT Allahabad is too a good institute, mainly focussed in Research and Development in Information Technology. In case you want to go for a Job post Graduation, do prefer IIIT A over MNNIT A, as it is one of the Top 10 Institutes in the Country at Par with Top IITs, BITS Pilani and NIT Trichy when it come to employer outlook and placements.