ORIGINAL: downunder
The "big" difference in plugs is the heat range.
The basic thing with plugs is that the one that gives the highest revs is the best plug (regardless of brand or heat range).
Not entirely correct IMO speaking about RC engines (not CL or wide open throttle). Plugs can have the same heat range, give similar top RPM readings, but one plug will allow a lower reliabel idle and a better transition over another. I believe this may be due to more or less winds, with varying thickness and metal content of the wire.
AJC