Akai-san: I do not know if a general rule really serves a purpose, unless you go behind it. Dan says this Kohaku is male, so I'm sure it is. I nonetheless referred to it as a "her" because that is the impression these photos give me.
Males are said to finish early and go down hill quickly, with none of their colors being as good as on females ... the Shiro more yellow, the Hi thinner and prone to breaking up after age 5. Maybe these types of generalities had more than a little truth in them decades ago. I think it may now be better to say that within a particular bloodline the females are generally superior to the males in body volume, length, coloration and likelihood of providing enjoyment over more years. If you look at male Kohaku bred at Torazo, the thick Beni is superior to what you will see on females at many (and maybe most?) breeders. The Torazo males lack in body volume, but not in their Beni, and seem not to grow much past a puny 28".
Hasegawa males are typically fairly scrawny, but the reds are bright against very white base color. I have the impression they retain their good looks a long time. Momotaro is setting a different standard with the focus on jumbo tosai. It takes a lot of genetic potential for a koi to be grown to 14"+ in their first year and have the colors look like anything; but Momotaro is spawning so many sets and culling such a high percentage that it is hard to imagine where it will lead; and, as pointed out in another thread, is producing some males of substantial size and volume. with excellent Beni.
So, yes, females are "better", but not all are better than all males, and I'm not so sure we do beginning koikeepers a favor by focusing on the sexes. Focusing on the pigment is what is important, although a very difficult thing to study and understand. Maybe in another 10 years I'll begin to recognize what I see when staring at Hi.