I am continually asked about plugs and heat ranges. For all, except race engines, I use and have always found the Bosch plugs to deliver the best life and service. For race cars it is hard to get a cold enough plug in Bosch so most of the time I use NGK.
Personally, I go by the VW factory recommendation as far as heat range and length of use. Naturally, this is based on the 6.6 to 7.0 CR recommended for all air cooled VW engines in relationship to today's fuel/octane. In the sedan, Karmann Ghia and Type III, I use the standard heat range GB 626 which is the 145T1 or W8AC for copper core.
The bus should use a colder plug, GB 626A, 175T1 or W7AC, as it is always working harder pulling the extra weight and pushing the flat frontal area. In fact, any time you are continually running at high speeds or up long hills with any VW you should go to the next higher number (colder) plug. Such as from 145T1 to 175T1 or 175T1 to 225T1 (GB 626B). A colder plug runs cooler and takes more heat before breakdown but will foul much faster if not run hard all of the time.
With higher compression ratios you must also run a colder plug. The higher the CR the colder the plug which must be used. This is from my experience of combinations and not from any set in stone policy, so deal with the people you get your parts from to select the proper heat range for your engine's application.