I am eligible for admission in Old IITs four year B.Tech Course, If I would like to go for M.tech admission, I want to know which option is the best either to go for B.Tech or M.Tech? Kindly advise, which is prefereable. I am confused.

First of all M.tech is a master's degree for which you need to be a gradute prior to applying for the admission in M.tech and if u've already done your graduation which is not B.tech but some other degree like BSc or something like that you should surely go for M.tech and not go for B.tech which would be sort of wasting your time doing under graduation all over again.