Generally, I think you would find that you hit the ST-H higher with more spin. This is due mainly to the cg location and the overall head design. The more traditional “hybrid” design promotes more trajectory with the vertical cg being further rearward and usually lower. The HD-I was designed to be just what it says, a Hybrid Driving Iron. Produces a little more of a driving trajectory, with less spin. The head weights, the weight of the shaft to be used and the fit of the player will determine the best length for each, but generally, since the head weights of the H2 and the 3H are only 2 grams apart and everything else being equal, would indicate they could be built to the same length. Hope this helps.