What is the difference in meaning about the words ("line" and "range") used as follows:
"a product line" and "a product range".
If there is no difference what is the most natural way of expressing a set of products?
"Product line" is generally used to refer to a set of related products (especially the products manufactured by a particular company). For example, if your company makes pet food, one line of products might be cat food, and the individual products would be a 5-pound bag, a 10-pound bag, and a 25-pound bag; your dog food would be a different product line.
A "range of products" (not a "product range") is what a store would generally have to offer, meaning a variety of different brands, types, and sizes of various products.
If you were a store owner, you might say something like "we sell a wide range of products, including Company X's entire product line."
There may be other ways to express a set of products, depending on your context.