I went through the same decision process in buying a tandem recently. There is a wide diversity of opinion out there and not much of what I would characterize as measured data. My not-so-scientific conclusion: go with 26" if your riding will consist of significant off road (gravel paths, fire roads or worse) or if the captain needs help regarding top tube height. Otherwise, 700c tubes/tires are everywhere and there are certainly tires available for 700c that can be used in mild off-road conditions. Our tamdem will take up to 38mm tires and the 40 spoke rims are pretty stout. Also, my wife didn't like the look of the 26" wheels!
There is the argument that for world tourers 26" gear is more available, but I don't know if that's important to you. In our case we're sticking to the US of A.