What should I opt for, mathematics and computing at IIT Dhanbad or computer science at IIT Palakkad?

You should choose the branch in which you are really interested. Here are a few factors which may help you in choosing. 

• IIT Palakkad is a fairly new IIT, whereas IIT Dhanbad is an old reputed institute with an extensive alumni network.
• If you are genuinely interested in Mathematics, then you should definitely go for MnC. I have some friends in CSE at IIT Dhanbad and their curriculum does not cover Mathematical topics much.  
• IIT Palakkad being a new institute will need time to offer infrastructural facilities at the same level as IIT Dhanbad. 
• Another thing that you need to note is that MMC is an integrated course with a 5-years duration. While CSE at IIT Palakkad will be a Bachelors’s degree or a dual degree. where you need to study for a minimum of 4 or 5 years. 

Hence, it all depends on you which branch you are more interested in and which will be easier for you.