B.SC. Nautical Science 3 year pre sea course, for class 12 students to join as Deck cadet on a ship. This course consists of 6 semesters, and provides a better insight into the job than the DNS course. After completing the pre sea training and receiving the B.SC degree,cadets can join a ship and can raise to the position of captain in 8 to 10 years.

Where as a Diploma in Nautical science is an undergraduate course of one year. This consists of two semesters and is also for class 12 students to pursue a career as Deck officer.

As for deciding which is better no one can answer that question.  Both B.SC and DNS have their own set of drawbacks and advantages. You should analyze yourself better which course suits you better.

By choosing B.SC you get to experience a college life, in these 3 years the college will help you build right attitude to tackle all the challenges. As both are Degree courses, the advantage in B.SC. degree is you can quit the job and can be able to enroll yourself in other PG courses. On the other hand,  if you studied DNS you will be stuck until you clear your MMD exam. While coming to advantages in DNS, the investment in terms of fees is less and you can start earning early also your job is secure.