I personally like the Black PC Engine Duo; I have an Arcade Card Duo, and 2 Avenue 6 controllers. I dumpster dove for the 30" CRT I play it on. As a result have no use for an RGB mod atm.
The original PCE Duo's more often than not need a soundfix; I have performed this repair on 60% of the units I have owned.
I've never been a fan of the R or RX despite their better capacitors and relocated power supply.
but just IMHO. The RX controller is really second-rate and sticky compared to the Avenue Pad 6 .
If you want reliability, collectability, and cool-looking go for a boxed Duo-R and get yourself an arcade card, a tap, and a few Avenue 6 pad for the fighting games.
Don't pay more than $175-200 for the base boxed unit
before shipping though. Some folks are selling RGB modded units on eBay for around that price.