Hello Debodeep, Both CSE and ECE are very good branch and have very good future growth and opportunities for you. But I will advice you to pursue the branch of your passion and interest. It will help you enjoy your course and bring the best in you. Many of the subjects of CSE and ECE are also common. You can always opt for doing major degree in one course and doing minor degree in another stream.
You can also switch to CSE subjects for M.tech after completing your B.tech from ECE.
Contemplate over the branch and your future and make a wise desicion.
For more information about switching branch refer http://www.inspirenignite.com/how-to-change-branch-in-m-tech