Are 18K nibs better than 14K?
In a word, no. When one finds an older pen with an 18K nib, it is almost always either French-made or made for the French market, where 18K was the minimum gold standard. Where nibmakers were free to choose their alloys the USA, Britain, Germany, and Japan 14K (sometimes also going up to 16K or, less often, down to 12K) was the overwhelming choice. A nib's qualities are determined as much by how it is shaped and worked than by its metal content, but in general 14K is more versatile and more capable of being worked into a springy, resilient form.
Note that most top American penmakers did not mark the gold content of their 14K nibs unless they were destined for export. American law did not require such markings, while almost every other nation did and still does.