Video output is all pretty much the same from TG-CD to Duo-RX.
I'd go for a Japanese Duo-R or Duo-RX as those seem to offer the best value for your buck, and are less likely to have problems. The first-gen black Duos (both US and JP) are prone to have bad audio systems, and while this is fully fixable, it's probably not something you'll want to deal with unless you're technically handy.
Regardless of what region console you get, you'll have to get it modded to play the opposite region's carts. The CDs are regionless.
Why I'd stay away from the original TG-CD: it's big, it's loud, you'll need a seperate "System Card" to play CD games, and it's slightly slower at loading CD games when compared with the Duo. Pros of the TG-CD: standard A/V connection, more robust laser assembly.
Duo-R/-RX cons: cheaper optical unit than the TG-CD (more prone to calibration drift or complete failure), proprietary A/V connection. Duo pros: sleek, low-profile, quiet, "System Card" is built into the BIOS, and a bit quicker at dealing with CDs.
For me the Duo-R pros outweigh the cons and outweigh the pros of the TG-CD. And you can avoid one of the cons, problems with the optical unit, by simply not playing CD-Rs in your console.