Look at the distances and where you want to turn off. If you have to go to the end of the runway, F3 may be useful. If there is a 10 HWC, F3 landing distance is going to be similar to FF with no wind. The idea behind the fuel savings is that in a big company, all those 8kgs add up to a significant $ sum over a year. If you are landing on the limits, then FF allows a bit more room for error.