If you already have a working tg16+cd there's no point in gettting a US duo. Save the money and get a PCE duo so you can play Japanese games and have a dependable system.
This! The Turbografx and CD unit just look awesome. AND, most people are more freaked out by it than the Duo because it looks so alien to them. Cracks me up at times peoples reactions to this stuff.
I use lots of different hardware because it is fun for me: TG-16 + Booster, TG+CD, PCE, PCE+SCD, TurboDUO, DUO.
I still use my TG+CD, but I've been using PCE+SCD slightly more (I tend to play a lot of Japanese HuCards).
(1) TG+CD = awesome (if unappreciated) aesthetic beauty*. Proudly use that beast. To play audio CD's, you don't have to turn on the TV/monitor, you simply use the buttons (LED displays tracks). Of course, you folks might not even use CD's anymore...but old fogies like me do.
(2) The footprint of the DUO is not significantly better than the TG+CD. The main advantage of the DUO is when you have limited VERTICAL height...loading/unloading CD's is easier when the CD cover opens fully.
*I know I'm in the minority when it comes to appreciating the aesthetics of TG-16 hardware. At least a few folks here dig it. Usually 99% deride it