I haven't ridden either. But my bias would be toward the Aluminum frame. Stiff is good, when it comes to Tandems. A flexible tandem frame results in "stoker wag" and is not a good thing.
In general, Tandem frames made of oversized Aluminum tubing tend to be stiffer. Hence my bias toward the Aluminum frame.
However, how any bike actually rides is depedent on more than just frame material. Thus, if possible test ride both, and go with the one with the best ride/fit.
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You could get lost and die.
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