Be careful... the description comparing W Shapes (or Wide-flange, apparently what you guys are calling H beams) to S Shapes (or Sloped-flange, apparently what is being called an I beam) is like comparing an apple to a motorcycle. There is no fair way to easily completely explain the difference in reactions. Most of the commercially available splitters are using some form of W-flange steel. The MAIN reason for it is that for a given WEIGHT of beam, you have more lateral and torsional strength in a given beam depth. S-flange shines in vertically loaded applications, such as overhead crane trolley beams. The big discussion should center around how HEAVY of a w-flange to use. There are 2 w-flanges that are approximately 8" high x 6 1/2" wide. One weighs 28# / ft and the other 24# / ft. While there is only 4# / ft difference, it translates into much more stiffness than the 17% increase in weight would indicate.
Personally, I am opting to use a MUCH stiffer beam for the splitter that I am collecting materials to build. I have a length of w8x67 waiting for me. It is 8"h x 8"w with flnage and web thicknesses TWICE the "standard" splitter beam size.