The gravity dual flush I have the most experience with is the TOTO and the Caroma.
Small water spots, and lot's of uncovered bowl area.
The TOTO stays the cleaner of the two. Water depth is fine for what is needed though. I sold a pressure assist dual flush for a short while, but it was a disaster.
You may want to look at the standard flush Watersense bowls. They use 1.28 gallons, get the rebates where available, have more water in the bowl and may come with a cleaner washing glaze. I sell a lot of the II series in the TOTO line for that. Double cyclone rinse, Sanagloss and they are WaterSense.