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.

くない and ではありません、ではない(じゃありません、じゃない)maybe くないuse for negatif いけようし(さむい、あつい、あまい、にがい、etc) Ex:さむくない(not cold) Then ではありません、ではない、じゃありません、じゃない use for negatif なけようし、めいし(すきな、ひと etc) Ex: すきではありません、ひとじゃない etc Hope can be help