For me, to fill up a big tank, its way economical to get NSW. Plus thing, its already sea water, no need further mixing of salt unless the salinity is not enough based on your needs. And with live sand and live rock, cycling is much faster.
Unless you want to have the parameters (numbers) like what was displayed on the salt container, and assuming they do give out the parameters, and don't mind spending $xxx for distilled water to fill up the whole tank, why not?
I had used both ways. Still prefer the cheaper and effective method to fill up a newly setup tank.