650B is considered superior for non-racing by many, for good reason: They are bigger than 26", so the bike handles like a "real" bike, but small enough to make a very strong wheel. 700C wheels on a touring bike or a tandem really are not the brightest idea, except that they are easy to sell, and parts are easy to procure. (I, too, have several tandems and touring bikes with 700C wheels, and I know that they can work fine.)
My experience with 26" is somewhat limited (a Bridgestone XO-2, a few tandems, all of which are off-topic, I know!), but based on that, I very much prefer 650B over 26" and smaller sizes (an even-more off-topic, even worse Bike Friday). And with the current state of repair of the roads in Seattle, which is not too different of that in Europe after World War II (they had tanks grinding them to bits, we have SUVs and studded tires), I am seriously considering switching to 650B on my performance bike. The ability to glide over potholes and bumps is amazing. Yet we found during Paris-Brest-Paris that the speed of our tandem (rolling downhill side by side with other tandems with 700C wheels) was not inferior. (I know that roll-down tests are affected by many factors, but it was surprising that a several teams of two short, stocky men on 700C tandems rolled downhill slower than our 650B bike with two relative tall, relatively skinny people. While the handlebar bag may have helped us aerodynamically, I deduct that the tires didn't slow us down...)
By the way, I am not sure whether there is a "critical threshold" between the tire sizes of 26" and 650B (as a very experienced tandem team from Boston with lots of experience with both sizes once claimed), which makes one handle great, and the other mediocre at best, or whether it is that all 26" bikes I have tried were poorly designed. It may well be that great bikes can be built with 26" wheels... and I just haven't ridden them. (And I think 26" wheels existed in Britain on lightweight bikes during the CR timeline, but I gladly stand to be corrected on this.)