To elaborate on floorless coasters vs. sitdown coasters, although they may look the same, the heartline of the trains are a little higher up on the floorless ones, resulting in minuscully different design of elements such as zero-G-rolls.
That's not the crucial difference, though: What makes them different purchases for the customers would probably be the mechanisms required for lowering the floor in the stations, as well as evacuation procedures. This would influence the design of walkways, the complexity of maintenance, possibly (but I'm guessing) certain safety regulations such as "no flip-flops", etc.
As for Hypers and Gigas, they're functionally the same rides. Okay, at some point you'd have to take aerodynamics into consideration on a slightly different level, but mechanics-wise, Gigas are just bigger Hypers. Likewise, Hypers can be considered bigger Megas, which are bigger... well, whatever we call those, but at some point the customer base will be so radically different that it's not worth it to group them together any more, from a marketing perspective.