Tandem enthusiasts can give you good advice on the various advantages and disadvantages of different frame materials and designs. However, you really should ride before you decide.

That's not easy to do, because tandems are usually few and far between. However, if you work at it you will be able to get some seat time and get at least an idea about what type of tandem is right for the two of you. You can probably get some test rides at these dealers:

http://www.crank-2.com/store/ (San Francisco area)
http://houseoftandems.com/ (Houston area)
http://www.precisiontandems.com/index.html (Kansas City area)
http://tandemcycleworks.com/ (Denver)
http://www.tandemseast.com/ (South of Philadelphia)
http://www.tandemsltd.com/ (Birmingham)

Another very good way to learn about tandems is to find a bike club that has a good group of tandem riders. There are only a few such clubs. We ride with the Orange County Rebels in Orange County, California, and there about ten regular tandem teams in the club. Of course, they probably wouldn't loan you their tandems the first time you meet them! Once you get to know the members, you'll be able to get a lot of first-hand advice and maybe a little seat time.

Good luck!