VA health benefits and Medicare are separate systems that do not coordinate the way two insurance plans normally would. VA coverage generally applies at VA facilities and for care the VA authorizes. Medicare applies to care outside the VA. Having one does not make the other unnecessary.
If you only have VA coverage and you need care at a non-VA hospital, in an emergency far from a VA facility, or from a specialist the VA has not authorized, VA benefits may not cover it. Part B is what covers care in the wider system. Declining it means the late enrollment penalty applies later if you change your mind, and that penalty is permanent.
VA pharmacy benefits are considered creditable drug coverage, which means having them generally protects you from the Part D late enrollment penalty. Some veterans still add a Part D plan so they can fill prescriptions at a retail pharmacy from a non-VA doctor. Whether that is worth it depends on how you actually get your care.
The real question is not which is better. It is where you actually go when something happens, and whether that place is covered. That is a short conversation, and it is worth having before you need the answer.
General information only goes so far. What is actually offered where you live, and whether it fits your doctors and prescriptions, takes about ten minutes to check. No charge for the review.