I run a Range Rover on LPG and it is well known that plugs are pretty critical when running LPG. A set of standard NGK plugs are about shot in 10,000 miles but I've had NGK Iridium plugs that have still been good after 40,000 miles. So, if you are doing lots of miles, then the extra is worth spending. If you aren't doing the mileage, then it probably isn't worth it.