I only have experience with a Turbo Duo and a Super CD attachment. Both work fine but I am a little dissappointed that the SCD is not as fast as my Turbo Duo. Nod has proven this to be the case with all CD attachments being slower than Duo's.
After speed, you considerations are price and reliability. I've heard lots of strong endorsements for the TG16 CD player and the Duo-R/Duo-RX both being real troopers (The R and RX are basically identical).
Many people with Turbo Duo's and PCE Duo's never have any problems, but D-lite or someone did find hard evidence of the heat sinks in these units having the potential to cause sound problems. Someone also made the argument that the CD technology at the time was so primitive that you're best off getting the most recent system possible, which again, would be the R/RX.
Speed, reliability and price would lead me to buy a Dou-R if I needed another system. On the downside, they're not black and the plastic tends to yellow over time, like a NES or SNES.