I've got both. I use my Duo-R as my main gaming rig and the briefcase setup as a back-up.
I didn't intend on owning both, but when I first got the itch to delve into the unfamiliar world of the PC Engine I ended up grabbing a white PC Engine console as they look so darn sexy. At this stage I had little knowledge about PC Engine hardware, it's not an easy thing to work out when you're new to it all and I didn't even consider the CD side of things. Already owning a HuCard only console I really didn't see the point in investing in a Duo, I came across a cheap IFU-30 so I grabbed it. Took me a while to find a working CDROM2 unit at a realistic price but eventually found one and pounced.
Years later I couldn't fight the urge to obtain a Duo so I did. Mainly got it for the convenience factor and now use it as my full-time gaming rig. I'd never sell my interface unit though, just too hard to come across them in working condition.