The only reason to go Duo R instead of Duo RX as far as I know is the yellowing of poorly cared for R systems. The RX doesn't seem to yellow. I bought my Duo R on Yahoo Auctions Japan complete in Box (and apparently unused, though that wasn't listed) for $220 about a year ago.
NIB Hori Commander PC controllers seem to go for around $45, and I've seen them in good shape for $35.
How do the Hori Commander PC controllers (2 button) compare to the Hori Fighting Commander PC (6 button). I like the look of the Fighting Commander pad, but will probably stick to the Avenue 6x2 for parody when playing with a whiny friend lol
I like them; they aren't necessarily worse or better than standard NEC pads. It's Hori during their glory years, so they're as good as 1st Party Nintendo. I personally find the rounded Hori DPad to be a little easier on the thumb during intense shooting games comparative to the NEC pad. If you're wild about the Nintendo D-Pad cross, they might be your favorite PC Engine option.
In general though, I personally prefer the Avenue Pad 3: the II and I button feel like they're in a more natural location as opposed to the 6 button options, and I love the ability to turn III into Run or Select depending on what I'm playing. Way more comfortable for the item crashes in Rondo of Blood. You can also assign Run to III and get a "slow-mo" pausing mode with its turbo switch, if that's your thing, I think. I've never bothered with that myself.