Having ridden all sorts of frame materials (steel, alu, Ti, Ti/carbon, full carbon, bamboo); prefer the c/f.
Have not yet ridden Magnesium tandem.
You get spoiled quickly on the carbon road feel (less vibrations, weight).
We've done a bit of writing on tandems and tested 30-some brands/models. Takes a lot to impress stoker Kay and her attitude after decades of doing this was ". . . not another one!" . . . when we promised to do an article on the Calfee several years back.
She was "impressed" (and she is not easily 'impressed!') with ride, ease of climbing and weight. Impressed enough that when it came time to order a new twicer she readily agreed to carbon fiber.
Friend/builder Bob Davis at that time gave us his prototype ZOna c/f tandem to test and from there on it was an easy decision to have Bob build us 'a second prototype' that fit our body dimensions.
We've owned three other full custom tandems and were very happy with all; our mantra is to buy the best you can afford and after 50,000+ miles on that twicer, we design a new one.
After riding the Zona c/f tandem for a 3-thousand miles, we switched back to our custom Co-Mo that had 57,000 on the odo. Rode it for a couple weeks while we had some changes made to Zona (re-clearcoated and the lugs painted black/gold; internal cable runs changed to external; replaced D/A STI with barcons).
Stoker immediately commented on the difference in the ride. Yes, she'd gotten spoiled on c/f!
Currently have 18,000+ miles on our Zona tandem and yes, it's a great investment/ride!
Have not experienced any amplification of bike noises/rattles as TG has.
As you noted, you would not switch back to metal frames on a single after experiencing carbon fiber.
Same goes for us with a tandem.
A c/f tandem can be fine tuned by the builder by doing the carbon lay-ups to suit the riders just like a metal frame builder can mix/match tubings to achieve a desired feel.
Alu tandems have a harsh ride compared to quality steel; Steel has a harsh ride compared to carbon. Most stoker want a suspension seatpost . . . but on c/f that can be eliminated.
Have seen/lifted, but not ridden, a Ruegamer "Ubertandem" and was impressed with the quality of workmanship/weight (sub-20 lbs ready-to-ride on a small size racing tandem).
Either Calfee or Ruegamer would be a great choice.
Hope this helps!
Pedal on TWOgether!
Rudy and Kay/zonatandem