Don't know why the Rimfire is better, it just is...
The manufacturer of the Rimfire plug is Paul Knapp...Paul is a perfectionist who owns a large machine shop in Arizona...He builds miniature gas engines as a hobby...He made a model Bentley radial...The NGK plugs didn't last very long in his engine...He appreached Champion for permission to make an exact copy of the 1/4-32 plugs they quit making when glow plugs came out...They declinded...He made a close copy...Myself and a few others were around and got involved in the testing of prototypes...One was a surface gap, no side electrode, but was apparently too cold so it was dropped...The current Rimfire was the best, and still is...
He makes many different sizes of VERY small spark plugs, now marketed by another company, Marvin and Morrison(I think)....
The Chinese version of the 1/4-32 plug is quite good, just doesn't last as long as a Rimfire.
One of the problems in the past was a good shielded cap for the ignition..Bill at C&H makes the best one but it's a labor intensive thing...RC EXL has come up with a pretty good cap, not as good as Bill's but adequate....And less $$$.
If you get an engine with 1/4-32 plugs be sure to take some spares along when you go flying....
FWIW, Dennis Crooks used 8.8 Husky Challenger inline twin engines in his 195 mph scale P38 unlimited race plane...Each engine had two C&H ignitions and four 1/4-32 Rimfires in it...I think they changed each plug and the end of a heat race..8 plugs at a time, $$$