Off the top of my head the Axial 3 is a two piece binding (see Look SPX10) and the Rockerflex (Look SPX12-18) is a one piece binding designed for the R21 plate. SPX 10 works on R20. The one piece binding is heavier, but it allows the ski to flex naturally under it. If you are looking for bindings for you, go with the R21 and Rockerflex. If this is for slaloms for a junior athlete and the primary goal is to keep weight down, go with the two piece. Otherwise Rockerflex is your only option. The setup looks gas pedaled, but really it is just compensating for the thick heel piece. I think U14 and under we used the two piece for slalom, and Rockerflex for everything else. U12, particularly lighter athletes, were two piece and R20 for everything.
Make sure you also order the lift kit with longer screws. You are not going to be able to find screws for the toe at your local ski shop. Lift kits come in 1, 3 and 5 mm. Most of our skis were about 3 mm toe down with the R21 and we used stacked 1 mm plates to adjust.