1. You can use normal spark plug but need to change it every 10k or 20k (based from my myvi). Up to you and it your choice to been made. Just calculate which one is more money saving.

Errr....personally i am using Denso iridium spark plugs in my Myvi 1.3A.
FC wise, i see no major improvement
Pick-up wise, maybe slight improvement

Good thing i like about it is that its worry-free for at least 100k mileage, and maybe will last longer than that lar...
Coz i always travel long distance from south to north, 750km in 10 hours drive, so i dont wanna worry halfway highway my spark spoiled coz i forgot to change it.

(Got one bad experience in my kancil, 1 spark plug spoiled and i ALMOST kena conned by "technician" as he said need to tow my car due to piston problem, then police petrol asked my drive slowly to nearest exit. The shop change spark plugs and air filter and changed as normal only, phewwww) - due to that time baru got license, car noob and know nothing about car but to drive it only... 
So eventually, it is up to ur needs lah...
If u feel worth it, then can install then let us know the differences in City ?
Research shows iridium plugs dont really improve much of FC and performance unless u driving a racing car 

iridium spark plugs can last at least 100,000 km. i've used them until 120,000 km and still in good condition. so u still save in the long run.