OFAC is more accurate that OFAB which is why Ford replaced it. OFAC uses a frequency based MAP Sensor whereas OFAB uses a more standard voltage based signal IIRC. Consequentially you can upgrade to a 3-bar on OFAB wehereas with OFAC you are stuck with Fords sensor.

In the process of refining the system the introduced a number of limitations that are difficult to get around. Hence if you are planning to go for big power then OFAB is more suitable, but for less than 200bhp theres not much difference tbh.