Between the 1.5" and the 2.25" model, are the V shaped ends a different size? I have a double layer of drywall situation I need to adjust for.

The height positioning feet for the support bar, are designed to work with ½” drywall, and are intended to set the proper height of the box. The legs for the 1-1/2” box are shorter than the legs intended for the 2-1/8” box. The bar for each is the same, and depending on the thickness of the individual drywall layers, it may be worth considering both, and using the deep box, with the shorter legs, which would allow for an additional 5/8” in drywall thickness.