Let's first observe it without です
塩は甘くない （casual Japanese (used among friends, most family, and younger people than you usually)
塩は甘くありません (polite Japanese (used when around people older than you or you consider superior)
If you add です after ない it makes it polite too so they are almost equal in value but I consider the first sentence more casual overall. Some (young) Japanese people might forget to make their sentences polite if in the mist of conversation (or like were just talking with their friends and using the casual forms a lot) and just throw a desu on at the end if they are about to forget. That's how I interpret this subtle difference but this is just my opinion. lol
Hope it helps.