Even though both courses end up making you a master, there are many considerations you should take before choosing one of the two.

DNS is a 1 year course and BSc Nautical science is a 3 years course. Further the below will be the main consideration for deciding which course you need to opt for to get into Merchant Navy

1) What is your career aim?: In both the courses, you get your BSC degree, in DNS you need to complete your required Sea time(18 months) and in BSc you complete the BSc degree in college itself (3 years)and then go out to ship for a shorter period(12 months) to be eligible for 2nd mates exams-If you don’t want to progress to sailing as master and want this BSC degree for doing your MBA,then BSC is better ,cause in DNS you need to complete your sea time and then give your 2ndmates exam and then you will get your BSc degree.
2) Which institute you are planning to DNS/BSc-This makes sense because it will depend on how fast you will go on board for your training, if you join an institute without placement you may even take 4 to 5 years to complete your 18 months’ sea time, considerable delaying your time to clear 2nd mates. In case of BSc, the sea time required is only 12 months and you already have a BSc degree and can do some job if your training gets delayed, god forbid. Most of the companies also prefer BSc candidates as they need to provide them lesser time onboard for training.
3) what is your financial back ground: How deep is your parent’s pockets is another criterion you need to consider as DNS course would be much cheaper compared to BSc and if you doing a course from a good institute you need not pay anything extra for your placements
4) Future trend: Future trend of shipping is moving towards more of technology and slowly the longer BSC will be more preferred rather than the shorter version of the Pre sea course. However, this has some time to come may be later in this decade, so if you are reading this in 2022, it may not make much of a difference in the context of future trend.