MBA/PGDM

MBA and PGDM are both almost similar degree with only nomenclature difference. You can choose any for higher studies.
Master in Business Administration (MBA) is two years degree course in the field of management. In India MBA degree can be awarded by any recognised university or by a Business School affiliated to any university. Post Graduate Diploma in Management (PGDM) is a two years diploma course offered by the institutes that are recognised by the All India Council for Technical Education (AICTE) and which are autonomous and not affiliated to any university. It is not mandatory that a PGDM is equal to MBA. PGDM institutes accreditation by Association of Indian Universities (AIU) makes an institute's PGDM course equivalent to an MBA.
There are a large number of similarities between the curriculum of both MBA and PGDM. An MBA institute follows a fixed curriculum as it will be based on the university guidelines which occasionally may not be aligned to industry requirements. Autonomous institutes do not follow university standards, so it is have the flexibility to align its curriculum according to industry standards and change in the business environment. Some of the PGDM from IIMs, XLRI etc has more value then MBA. However MBA of FMS DU, IIFT, etc. have similar value as IIMs. You are advised to chose Institute based on its ranking not on nomenclature of degree.

As a mechanical engineer, you are eligible for all the branches or specialisations of MBA. There are multiple functional specialisations in MBA such as finance, marketing, human resources, IT, operations, SCM, logistics, operations, systems, general management or sector specific specialisations like travel & transportation, telecommunications, healthcare, banking, insurance, etc. One must chose MBA specialisation based on his or her interests, prior job experience and personality, skills and competencies.
Marketing is suitable for extroverts. If you have good interpersonal skills, sales aptitude, leadership qualities, good communication skills and have entrepreneurial attitude then you must go for marketing. Individuals with good numerical ability can do very well in the field of finance particularly investment banking. If you good at number crunching, have superior numerical ability, good data interpretation aptitude, problem solving and creative thinking and you are also a strategic thinker then you must opt for MBA finance. People with social skills and empathy are suitable for HR. To be become good HR manager you require skills such as organising capability, dealing with grey areas, good communication and negotiations skills, capability to handle changes, capacity to handle conflicts, etc.
Most of the mechanical engineering graduates prefer marketing or operations or SCM specialisation.