Short answer: You should be on the medium bindings.
Long answer: That much of a gap is going to make the bindings a bit more sloppy even if you really crank down the straps.
Bigger issue is that it might be difficult to center the boot on the board, given that Burton heel cups are not adjustable.
If you cannot get the boots approximately centered than you need the medium size. If the boot position is ok, then it is up to you to decide whether you can live with the extra slop.