To Start,

there are short and long plugs. Short plugs are PRIMARILY used in .25 and smaller engines. Long plugs are PRIMARILY used in .25 and larger engines.

Next there are regular and "idle bar" plugs. Idle bar plugs are RARELY used these days. They were designed for the older engines that had a "raised wedge" in the top of the piston for fuel/exhaust input/removal. The FEW instances where an idle bar plug MAY be used these days, is with an inverted mounted engine.

Now we come to the fun part - heat ranges. They have cold, hot and everything in between. SOME examples are an OS A3 plug is a hot plug, while an OS#8 is a cold plug. The heat range of the plug you use is dependant on a number of things. The primary item is the percentage of nitro in your fuel. Other factors such as altitude, temperature, size of prop/type of flying also influence the heat range.

The last "monkey wrench" in this mess is 4 stroke engines. They NORMALLY require a long plug equivelant to an OS "F" type plug.

To answer your question-
Check the instructions and see what plug is recommended for the engine you are buying and get a comparable heat range. Your LHS should be able to tell you which of his brand of plugs is comparable.

FWIW, MOST 2 stroke OS engines, .25 and larger USUALLY use an OS#8 plug (cold plug) with 5% - 15% nitro for sport flying.

Plug life and quality vary, so get some spares AND in different brands also. I have had plugs last 2 seasons, and I have had them go after a half dozen flights. I have had my best results with OS, Enya and McCoy brand plugs.

This is probably more info than what you wanted, but I hope it answered your question.