Iridium spark plugs are constructed of harder metal and have a higher melting point than platinum spark plugs, so they tend to last longer.
Some car manuals specifically call for iridium spark plugs, and, if that’s the case, you shouldn’t downgrade. However, iridium ones are more expensive upfront.
Platinum, in turn, lasts longer than copper spark plugs and is more expensive, though some older cars will require copper.