A pretty broad question really. But the answer in most cases will be dictated to you by the rules of your local/regional race series. You kind of have to run what the tech officials tell you that you must run.
As far as preference goes, most clubs or organizations (WKA, SKUSA, FWT), as well as Series (RMC, Rok Cup) will specify specific plugs that are LEGAL for use. They usually leave the temperature range up to the competitor.
Iridium plugs require less ‘voltage’ because the Iridium conducts better. It’s much harder than the electrode on standard plugs and as such tends to wear longer/better. The problem with Iridium plugs and karting is that if your jetting is not correct, due to the much smaller electrode, the plug can foul much easier. Additionally, Iridium plugs are typically MUCH more expensive than regular plugs.