Have ridden/owned a variety of frame materials in tandems/singles since the early 1970s.
Various steel alloys, heat treated aluminum, titanium, bamboo and carbon fiber.
Steel alloys are a good choice and available in the widest range of prices. Durable, comfortable.
Aluminum is usually stiffer/harsher, especially in the stoker compartment without a suspension seatpost.
Titanium is pricier, bit lighter/stiffer than steel and not as harsh as alu; the difference in the $$ did not offer us much more benefit.
Carbon fiber is much lighter and offered us a much more comfortable ride, albeit at a much higher price.
Bamboo we equaled to c/f in both in comfort and price.
Having said that, the builder, frame design/components can make a huge difference in how these tandems perform and appeal to you.
Mel at Tandems East is a great source for a big variety of tandems to test ride.
If possible do several back to back rides on different bikes; then do it all over again.
Asides from material, proper fit/handling is the highest priority.
And of course, you wallet will dictate what you are willing to spend.

Pedal on TWOgether!
Rudy and Kay/zonatandem