Personally I use L2 R2, I tried L1 R1 for a time, but found that you have to press really hard on them to get them to do anything, the triggers have a more gradual activation of the airbrakes...
On the other hand, L2 R2 for airbrakes makes it absolute hell to sideshift as you have to let go of the triggers completely before pressing them again otherwise the game just registers it as continuous use of the airbrakes. Can't tell how many times I've smashed into walls where I wanted to sideshift away but the game didn't let me.