Traditionally, an engine would be serviced based on tach time, because, well, that 's what folks had attached to an engine. Makes sense if you assume that the engine should be serviced based on "number of revolutions", which is what the tach really records.
I guess I never really thought about the fact that i am now using "real" clock hours, because I base my service intervals on my EFIS clock hours. But I also don't feel bad if I don't go an hour or two beyond the regular interval.
Paul