<snip>On the other hand, I'm pretty confident you could switch your wife's crank onto your bike and you'd get used to them in a short time and get to like them just fine. But it's a tandem stoker crank, and the pedal threads will be backward, so never mind that.
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You often mention that it's important to spend some time out of the saddle on all rides (I hope I'm not confusing you with someone else), and it's good advice. Shorter cranks do tend to discourage the rider from getting out of the saddle, and that's not a good thing. Anyone investigating 'ideal crank arm length' would need to keep this in mind. Whatever the ideal crank arm length is, it's going to be a compromise.
So far I haven't ridden a tandem on which normal L and R pedals didn't fit all 4 cranks normally. Specially threaded tandem cranks may be ancient history. In any case, I already spin 100 comfortably on the 175s, and am not about to give up the OOS leverage or the steep stuff seated leverage. I don't see what shorter cranks could improve.
We've now ridden a couple hundred miles with the 151mm stoker cranks. It became obvious that we were much slower on steep climbs where our cadence dropped below ~80. On the good side, Stoker likes them. On the bad side, she's cramped more with the shorter cranks, something I did not expect, but because of the shorter lever arm, she has to push down harder on the steep stuff to produce the same power. On the good side, she can stay up a little longer when we stand. On the bike mods side, I've put a 40T cassette on, so now our lowest gear is 26 X 40. That gets us up the 15% grades we could not climb with the shorter stoker cranks and a 34T cassette.
On our tandem, captain's ratio is 5.56 and Stoker's is 5.59. On my single, my ratio is 5.40. Stoker's previous ratio was 6.30.