Salt water has nothing to do with it. Either method works fine; it's a matter of what is convenient and cheapest for you.
So, what you should do is use PoolMath to figure out how much Borax/MA or Boric Acid you will need to raise your pool water from 0 to 50ppm. Then, you can compare the costs for each procedure. In my pool, ordering from Duda Energy (they are no longer called Duda Diesel) online and using boric acid was about $10 cheaper than using Borax (purchased from Walmart) and MA.
Also, you'll want to follow the procedure as outlined in the link in the previous post. The key is to get your pH and TA adjusted to the point where you'd like it to be and then add the borates. If you choose to go the Borax/MA route, then the additions of the chemicals need to be broken up into 2 or 3 batches with a good deal of brushing and agitation in between. If you use boric acid, you can add it to the water all at once by broadcasting it around the pool and then brushing to mix it in.
If you order boric acid, make sure you order the granular product not the fine powdered form. The fine powder will blow all over the place and float on the water surface before mixing. Granular works best.