have a search for it on here mate.

quite a bit of info

ofac is frequency based
ofab is voltage based

you can use the 3 bar cosworth type map sensors with ofab and hence potentially can be seen to have more tunjing scope.

ofac - iirc MSD in blackpool prefer the ofac system and can still map them for decent boost levels.

best speak to the people you intend to get the car set-up or mapped/chipped by