MBA and MCA are two totally different courses and have different career scope and prospects. After MBA, you perhaps have a wider career scope with Finance, Operations, Manufacturing, IT, Business and Corporate Industry in Managerial Roles. After MCA, your career scope mostly limited to Software/ IT/ Services Industry.

I would perhaps advise you to pursue a MBA Degree after B.Tech in Mechanical for better career scope and prospects. MCA is primarily for Non Engineering Graduates, interested in developing a career in the IT Industry, though after a B.Tech you can definitely apply for it.