If you think that research is what you what to do in the long run,then go for this,as not only do you get more opportunities for research and choosing which part of the branch you are interested in but also you can directly go for PhD after the completion of the course.

Since there is no exit option at the end of 4 years,and if do not end up liking the branch then the dual degree program can turn out to be a nightmare and waste an extra year.

If you are not sure whether you want to do research in the future,the best option would be to take a single degree(BTech) program in the branch of your choice in any old IIT. If you feel that Mechanical interests you but you cannot commit to research right now,go for BTech Mechanical in IITs where you are getting admission.