At current prices from most expensive to cheapest:
1: USA Turbo Duo
2: PC Engine Duo-RX
3: PC Engine Duo-R
4: PC Engine Duo (black and purple)
Since it release my favourite model of Duo has always been the original Japanese black and purple one. In fact it's my favourite console of all time.
Over the last 10 years the Duo-R and Duo-RX seems to be a more popular model. Not just because of the capacitors, people appear to like them more cosmetically, which I don't agree with.