(Disclaimer: I've only ridden steel tandems, but I have previously owned steel, aluminum, & carbon half-bikes.) Carbon is a much more comfortable ride than Al. That said, when I moved from the Cannondale R200 to my Specialized Roubaix, I noticed that while the Specialized' turn-in was sharper (& faster) than the C-Dale, it was also a lot more nervous (the term I'm using to describe the carbon frame's resonance). How much that tendency exists -- or doesn't -- exist in a tandem frame should be left to the long-experienced carbon tandem folks like Mark, Rudy, et al.