I'm not sure there is much of a difference anymore, but I believe initially the difference was speed, as an NPN transistor was much faster then a PNP, so if you needed faster response time, the NPN transistor circuits were more desirable.
Thanks. This is what I was assuming, but it was always "drilled" into our head that no welding was ever permitted. I think this was more to prevent anyone from simply "tacking" on a bracket or something that might jeopardize the vessel's integrity.
Anyway, thanks again.