The Duo R/X have the video and power cable ports in the back, which is a big deal to people like Joe Redifer.
The DuoR/X also have a much sturdier exterior than the regular Duos.
The regular Duo's HuCard slot cover can pop off and break easier that the sturdy permanent Duo R/X cover.
The Duo R/X lack the earphone jack the original has. They also stain and show dirt and wear.
The Duo R/X can run on the PSOne AC adaptor.
The Duo R is sort of a cream white with a white CD door button and a logo at the middle/center of the top. I prefer the look of the Duo RX, which is a more like a pale gray with a purple CD door button and a stylized logo at the top left corner of the top.