This question is subjective to the aspirations of any given student. If a student is interested in doing research in a particular field and is in no rush to join the service sector, he should go for an M.Tech from IITs. Since a GATE score is valid for three years, he/she may still be able to apply for M.Tech provided he meets the PSU's GATE cutoff.
For a student giving GATE exam only for PSU recruitment, studying at the IITs for two years makes no sense. He should join the PSU and focus on his professional career.