If I can keep one in my cooking with hypertension, which is healthier--butter or salt?

Butter or salt. If you have hypertension you should give up salt first.Butter is also not healthy as it is saturated fat, but it will not effect your bp, it may increase your LDL cholesterol.
There are options. Try using a product like losalt which is about 30% sodium and 70% potassium (which helps lower blood pressure) salt. There are delicious varieties of olive oil to use instead of butter. Olive oil (use cold or low temp cooking) has many health benefits. Then try clarified butter, called ghee, in tiny amounts if you still want butter flavor. It never goes rancid - will keep for years outside the fridge.