To choose a branch depends on your capability & interest. Both are the good branches & have scope in future.

Mechanical Engineering helps the students to understand the working mechanisms of heavy tools and machineries.  The branch applies principles of engineering, physics and material science for the design, analysis, manufacturing and maintenance of mechanical systems. The job demands supervision and planning of live projects related to thermal power plants, nuclear stations, generation of electricity, etc.
Mechanical Engineering have job opportunity in private sector and public sector while CSE branch doesn't give much options in public sector.

Computer science engineers are involved in many aspects of computing, from the design of individual microprocessors, personal computers and supercomputers to circuit designing and writing software that powers them.

Job openings for Software professionals is much higher in the corporate sector than in public sector. Professionals can join as junior programmer, database administrator, junior network manager, Data Analyst, Software Developer, Software Engineer, and Client-Server Systems Manager etc in the initial stage.

Good Luck!