If you want a system that plays US stuff its cheaper to get a TG16+CD over a Duo but the duo has advantages, notably 1 power supply. The Duo is expensive and most need(or will soon need) a redoing of many onboard capacitors
Japanese systems I dont know much about but the Duo-R is most recommended. I believe its the cheapest all in one systems. I think a PC Engine+CD is cheaper than a Duo-R.
If you want legal access to every game you need a SuperGrafx+CD(plays the 6-8 Supergrafx) with a region mod.
Theres good flashcarts for playing roms and translations and can save you from having to region mod. All CD systems play burned games but it is not recommended as it will burn out the lazer. If you need RGB(if you use an upscaler) the standard PC engine and Turbografx have external solutions for RGB but the CD+card systems need an internal mod(not too difficult, more info here). Overall as a system I like it a lot but I am a big fan of shmups, and that makes like 30% of the catalog. Theres a lot of junk on the system though.