Isnt the way to explain it more like
Arcade Pro: Anything with a HuCard slot and CD drive from Japan
Duo: Anything that says Duo in the model, from Japan?
Anyway MXS, the best arguments for getting an R or RX over a regular duo, like was mentioned, is the sound capacitor problem.
The only incentive for a standard Duo is if you prefer a black system.
and R vs Rx boils down to a minor color variation (whiteish, vs. grayish) and a pack in controller difference...
but, both pack in controllers are meh, and you should get an Avenue 6 pad anyways, so it doesn't matter much! O:)