There is an engine reviewer called Andrew Coholic which writes for FlyRC, reading some of his reviews you can note that there is not a significant difference among some brand of plugs, I believe that the only significant difference among the plugs is how hot or cold they could be and when is the time to use one or the other.
About the life span of a plug I have to say that I have engines with the same plug for over 4 years and still working good, sometimes it is good to change the original plug after breaking in a new engine but it is not a rule and the same plug could last very long.
I use OS F type plugs on my 4 strokers and A3 and A8 on my 2 strokers.

Happy landings.