I have a Dishwasher which is 12 Amps and 1.2KW and a Garbage Disposal which is 8.1 Amps and .75 Horsepower (575 Watts?). I want to run them on the same 20 Amp circuit, because running a new circuit would require a lot of patch and repair work, but also want to make sure I'm under the 80% continuous load limit.

It would seem from a Watt standpoint I'm under 1920 Watts (80% of 2400 Watts - max power for a 20 Amp circuit), but I'm over in terms of Amps. Which is more important?

amps X volts = watts, are you sure you are reading the labels correctly? Your amps add up to 20.1, 80% of a 20 amp breaker is 16 amps. Your wire and your braeker are rated in amps, #12 wire on a 20 amp breaker can do 16 amps continuously

We use one circuit for the dishwasher and garbage disposal. The high watts of the DW is from the dry cycle, so chance of both GD and DW dry cycle running at the same time is nil.

Plus the breaker can be loaded to 100 % since these do not run for more than 3 hours.

Neither of these appliances are a continuous load. Almost nothing in a home is.