I notice that, on Xbox at least, both RB and A are assigned to handbrake. I had a look at the controls and noticed that A is also used for answering the phone in the vehicle, so my question: is there any difference between the two handbrakes? I know on a bike there is additional braking controls (different buttons for front and back brakes) but presumably this is not the case in a car.
According to the manual, there is no difference.
A and RB buttons are both mapped to the handbrake, so it's just for convenience.
Personally, I alternate between the two based on what I am doing.
When I want to change weapons, X, and make an immediate sharp turn, I would use A.
Otherwise, I would use RB (especially if I had to answer the phone).
Hope this helps!