NGK Spark Plugs (my personal preference, BTW) has a bit of a blurb on spark plug design
Spark Plug Basics
The spark plug has two primary functions:
- To ignite the air/fuel mixture
- To remove heat from the combustion chamber
It is important to remember that spark plugs do not create heat, they can only remove heat. The spark plug works as a heat exchanger by pulling unwanted thermal energy away from the combustion chamber, and transferring the heat to the engine's cooling system. The heat range is defined as a plug's ability to dissipate heat.
The rate of heat transfer is determined by:
The insulator nose length
Gas volume around the insulator nose
The materials/construction of the centre electrode and porcelain insulator
A projected style spark plug firing tip temperature is increased by 10°C to 20°C.
So ... to answer your question, I would go with the recommended plug right fom the catalog for a "bone stock" engine. If the engine has been modified, the recommendation is to go to a "colder" plug.