From my personal experience getting verified on casinos and also on other kind of services, it would seem to me that passports have more value and weight when comes to making sure the casino can be sure you are summiting actual information which is related to you as a person and there is nothing fishy going on.
I have got that impression, because in my country, our national identity cards are very cheap and made of low quality material, they do not have any security element whatsoever. So in the eyes of casinos is better if gamblers can send them passport pictures, which follow a minimum of international standards and they are also harder to counterfeit when compared to identity cards.

I have used identity cards, but since I have managed to get a passport on my hands, I have opted to use it for gambling and non-gambling verification on internet.