The Ultimate and the Ultramax use the same porcelain.
The Ultramax was supposed to be their "HOTEL" version of the Ultimate. Toto figured the Ultimate worked plenty well enough for home use. I personally had one installed for two years, and it got heavy use with big crowds on weekends. It went two years before I needed a plunger, and then I didn't even know where to fine one.
The Ultramax maintains a higher water level in the bowl, with the fill valve set higher and the higher overflow tube.
Since the water leaves the tank quicker, TOTO used the red flapper with the hole in the lower portion to speed up the drop. It's rated at 1.6 gallons.
The Ultimate, with the lower water level, needed the slower closing blue flapper that was plugged on the bottom, and because the water was slower to leave the tank, it still uses 1.6 gallons.
If they only have the Ultimate, I wouldn't worry about it, they work fine. And if you think you really need more water, just hold the handle down a pause more.
The Ultimate is a little quieter, since the water flows through slower. If the Ultramax had never come out, I would be selling plenty of Ultimates. But like anything in marketing, you can never be too quick, too large, or too new. Even if the old dog has been herding the sheep just fine the whole time.