An I-Beam rod can handle high compressive loads while also providing good tensile strength. But the thickness and strength of the steel in the rod limit what it can safely handle. So performance I-Beam rods are typically made of a higher grade of steel (4340 or 300M), and often have a thicker cross-section in critical areas to increase strength.
H-Beam rods have a completely different design. An H-Beam rod has two large, flat sides that are perpendicular to the piston pin and crankshaft journal, with a thin center section in the middle. This makes the H-Beam design very stiff so it can handle higher compressive loads without bending.
Which is stronger, I-Beams or H-Beams? It depends whom you ask, and the relative weights and cross-sections of the rods. I-Beams can be just as strong as H-Beams, but H-Beams can often handle higher compressive loads than I-Beams with less overall weight.
Consequently, H-Beam connecting rods are often recommended for high torque motors that produce a lot of power at low rpm (under 6,000 rpm). Some rod suppliers offer H-Beams as their “entry level” or less expensive line of performance rods, and offer I-Beams for all of their high end racing applications. Other suppliers only sell I-Beams, and some only sell H-Beams.