The use of Iridium is only to increase the service life, that is the only thing even claimed by the manufacture denso in this case. And since the service intervals of our cars are not the same as abroad due to really harsh driving conditions and the way we maintain our vehicles with cheap spares, there is absolutely no point of using an expensive plug for very long service life. Just put in a simple copper tip and throw it away after 25K if you feel like, even after three such iterations you might not reach the cost that you would incur for iridium which are utterly useless in this case. All the thing claimed by here by the change of plugs are baseless, maybe the genius also got a fresh oil change when he changed the plugs and then calimed meri to gari badal gayi yar 
The following are stock plugs for a vitz 1KR-FE engine :
DENSO K20HR-U11
NGK LFR6C-11
BOSCH FR7KCU
I have a personal inclination towards NGK although I am aware that denso came as the OEM ones in vitz