I'm not familiar with "datcom" per se, and google comes up with 250.000+ hits, but I guess it's similar to a "hobbs" meter which runs only when the engine is running. It probably also runs linearly, meaning that if there's exactly two hours between engine start and engine stop, you are charged exactly two hours. Doesn't matter whether you spent those at the hold or in a fast cruise.
A tacho also counts the hours the engine is running, but is dependent on the actual RPMs. So while you're idling at the hold, it might take two hours or more for the tacho to increase by 1.0, while in a very fast cruise, that same 1.0 on the tacho might only take 50 minutes. Normally a tacho is calibrated to count "true" at the normal cruise setting.
What most, if not all, not-for-profit clubs/schools/groups do is divide the total annual costs, by whatever the hobbs/tacho/whatever increased over the year, and use that as the basis for the hourly fee to pay, again calculated based on that same hobbs/tacho/whatever. So normally, at the end of the year, it all averages out and there really is no difference.
However, if your flying style differs greatly from the other pilots in the group/club (e.g. you fly long and high IFR legs while the others just do short, low-level bimbles) you might have a relative advantage or disadvantage.
One advantage of a hobbs meter, I find, is that whatever is on the hobbs, can go straight into my logbook, as long as you move the aircraft a bit as soon as you started the engine, and don't wait too long before shutting it down once you've set the parking brake. Can't do that with a tacho.