As compared to the original Diamir, the DII strengthened the central rail and added some plastic shims to increase the lift by a few mm (and in the process also increased the weight noticeably). As compared to the DII, the DIII has slightly redesigned plastic housings for the toe and heel unit, presumably w/ the goal of adding strength (and also adding a small amount of weight). As compared to the DIII, the DFR has bigger shims for an additional 4mm lift, a slightly different plastic housing for the toe unit presumably to add strength (though it loses the capacity to attach a leash), and release setting to 12 (instead of merely 10). The DFR is somewhat heavier and more expensive, although this comparison is complicated by the DFR’s standard inclusion of a brake but NOT a return spring.
If you can find a DII in the appropriate size (don’t be tempted by a Long if you can fit a Regular, since you’ll be putting more stress on the longer rail), and can live with a release setting up to 10, I would go w/ the DII.