Actually, the two women share the basic horse-face look, though Diana pulled it off more daintily. Obviously, Charles finds horse-faced women appealing, so why shouldn't he think the horsier of the two was more beautiful? It's good for people to have different beauty standards, because it increases the total amount of happiness in the world, making it more likely that people will be able to mate with a person whose looks they love.
Their love is a tribute to horses, in fact. The two met over horses:
The Prince of Wales and Camilla Shand, as she was then, hit it off from the first time they met, at a polo match in 1970. But Charles was something of a playboy, and when he dithered and went abroad with the navy, Camilla married a longtime suitor, Andrew Parker Bowles. The two remained friends with the prince. Mr. Parker Bowles even took on the ludicrously named ceremonial post of Silver Stick in Waiting to the prince, while his wife took another traditional role - that of the prince's mistress.
Silver Stick in Waiting? The ways of the English are mysterious! But good luck to couple, presumably happy at last.