Which one is a better idea after passing an MBBS, MRCP or an MD?

It depends on where one intends to practice.

India needs a large number of doctors in coming years and decades. In India, unless one is practicing in a metro having an MRCP does not matter much.

The clinical exposure in India is far better than in the UK. The techniques, skills and protocols are better in the UK. If one intends to practice in a non-metro city or a town in India, one cannot expect the same diagnostic setups and protocol based practice as in the UK. The conditions are more conducive for a Doctor with MRCP in metro cities, provided you intend to work for a corporate set up. People recognise that it is a foreign degree and are willing to pay a premium to consult. In towns and smaller cities it does not matter much, people might even avoid you perceiving you are too costly or too out of sync with local milieu.

MRCP might be useful to get into national institutes and also to find work outside of India, especially in the middle-east and in the commonwealth countries.

An MD from a reputed institution is what one needs if one intends to practice in the town and smaller cities. MRCP will serve as an added advantage on for such practitioners. Second and third tier cities and towns require a lot of Doctors still and will take in a lot of specialists and super specialists in the future.