Why recommend a Duo-R over a regular Duo?
To me they are exactly the same console except in color. They have all the same features and capabilities. They both need the caps replaced although yes the PC Engine Duo, TurboDuo usually have more damage from the caps once fixed it's a non-issue from my standpoint as well.
I can see why recommending either of the console's, Duo, Duo-R over the regular TG16 or PC Engine as they include the Super System 3.0 in the firmware within the system and it's all one unit instead of two.
US or JP version of the Duo and the Duo-R are very viable options but the JP versions will be cheaper. Just make sure the console has had it's caps replaced or plan on doing, having that done to the console once you get it. Then make sure you get the mods you want if needed, Region and/or RGB.
Those are all factors to consider in the price. I would pass on the US version TurboDuo because almost no matter what you can get a PC Engine Duo for much less. Duo-R's are a little more than the regular Duo's usually but even if the system is a fully working system it's smart to have all the caps replaced just so you know you have another 25yrs without worrying about it.