Typically, an A-A consist would be more common for two units, especially on the NYC. The booster units would be added as needed for additional tonnage or longer trains, while the A-A consist would provide more flexibility in that the locomotives would not need to be turned at the end of a run. Furthermore, much like arch competitor the PRR, NYC would run booster units as part of an A-B-A consist. NYC followed a similar practice with passenger trains - A-A units on trains or if needed an A-B-A consist.
Where I have seen A units paired with B units only has been more common on passenger trains on the western roads. Santa Fe purchased A-B-B F7s in the 300 series for passenger trains while many of their passenger trains operated as A-B units. SP followed similar practices of A-B units for passenger trains.
One resource is to look at the number of rostered F3s in the A and B configurations and you will see that booster cab units outnumbered booster units by a fairly sizeable margin in most cases.
However, it is your layout so your practices are what matters most. I like A-B configurations for passenger trains as I don't need to set the unit to run in reverse in command for my 3 rail trains.
Hope that helps!
