The white Duo-R was sort of a 2nd gen Duo that was released as a PCE Japanese model only. The RX was basically the 3rd gen version. Functionally they are the same as the original Duo, but are generally known to be more reliable whereas the original black Duo (both US and JP) seem to be plagued with capacitor issues (fixable, but requires soldering). CD games aren't region-locked and can be played on either system, but hucards can only be played on their native systems (unless your console has a region mod done to it).
There are more of the Rs and RXs available than the US Duos, and they are generally cheaper, even after the pricey shipping from Japan where most of the ones on eBay ship from. They go up for sale here from time to time, if you stick around that might be your best bet.