Hey Purushotham Kumar Singh,

Marine engineering is an engineering stream which completely refers about shipping, industry , trade and management filed too. It is based on shipbuilding to tradinng. 

Merchant Navy is completely based on same as marine engineering, but is just as division of marine engineering referred as a sub part. This field is completely a commercial field. You can expect international trading from this field

The best one is based on you. Because both of them are equal. Because merchant navy is a sub division of Marine engineering