I have a Vespin and a Carlyle, and performance wise, I don't see a difference. These are not the newer II style. In theory, the Vespin should perform a little better since the tank is taller...the further the water falls, the more potential energy available. I wouldn't worry about a toto with the 1.28g flush...they work fine from what I hear. And, basically, if you change the flapper valve (get one without the float), it will use more water, if you ever decided it needed it (which it shouldn't). A one-piece toilet has fewer nooks and crannies that can get dirty from splashes, etc. It's a little intimidating tightening the tank down the first time to get it tight, but as long as you do it gradually, and evenly side-to-side, you shouldn't break anything. A one-piece is harder to set since it is heavier (this assumes you don't install the tank until the base is in place with a two-piece). Other than that, it's style, and preference. the one-piece costs more since if for some reason something goes wrong, they have to scrap more.