I only buy cloth that is otherwise inaccessible to the tailor. I don't think you generally save money buying cloth. For example, NSM didn't have access to H Lesser and Smith's in the beginning. There were limited runs of cloth from LL and etc.
All things equal, heavier has better drape, warmer, less wrinkles and presumably longer lasting (I haven't gotten there yet). IMO the differences in warmth, going from 13 to 16, is greater in flannels than regular worsted.