The entire Toto line boasts excellent-flushing toilets of the highest manufacturing quality. The Kohler line isn't so consistent. Given Kohler's advertising, marketing and public reputation, this is shocking, but true.
Let's review: Who's paying for the product? Who's living with the product? Who is leaving after installing the product and pocketing the profit? It's not like there's anything special about installing one brand of toilet over another. It's like saying that your contractor preferred Phillips light bulbs over GE bulbs. They both screw in the same so get the one you want. If he doesn't know where to get it, do a little research (like the research that found this site) and you can order one yourself. You can also call around local plumbing supply houses (the prices will be all over the place, so make at least 3 calls).
When I want X and someone tells me, "Well, we usually don't do X," my response is always, "I'm so glad to hear that you sometimes DO do X, because that is what I would like."