Hello Sarthak!

   First of all, congratulations on your selection to these institutes! 

So,CMI Chennai will provide you BSc. Hons degree while BITS Pilani will provide you with B.E. degree in Computer Science. The basic difference will be in  the job profile you'll be eligible for. Also, the placement statistics are very different for these two colleges. If you seek an immediate, secure future, BITS Pilani CS it is. B. Tech will provide you more technical exposure. But if you are more oriented towards research then you go for BSc. CS.

Best Wishes!