They actually don't both look the same -- the H and I describe what a cross section of the rod looks like if you were to cut through the middle of the rod and look at the end. If you orient the cross section relative to the way the piston pin goes through the rod, you'll see that the H cross section is rotated 90 degrees to the I cross section when the rod is mated to the piston. So they're quite different.
Others will have to weigh in on the pros and cons -- but here's the kicker. Properly prepared with high strength fasteners, the stock 5.0 I-beam rods are probably stronger than the block is -- so unless you're building a real butt-kicker with a Dart block, there's probably little reason to replace the rods with anything else anyway.