Howdy from Tucson, Rob!
Our money (and our butts) are on carbon fiber; we both agree: more comfortable than any steel, alu or ti tandem we've ridden (including, 'tana, Burley, Co-Mo, 'dale, Serotta, daVinci, Trek . . . and more). Great handling, easier climbing and much-much lighter! Our custom tandem weighs in, ready to roll, under 30 lbs. Currently have 'bout 8,000 miles on the odo.
Check out the ariZonaTandems.com website. He is introducing a (Gasp!) 24 pound tandem for 2006!
If you order before the end of this month, he'll build a custom fixed c/f stoker stem for free (a $300+ value).
Even if you're just a-lookin and not yet ready to buy, check it out.
Pedal on TWOgether!
Rudy and Kay/zonatandem