Franck's harmonium music sounds subtle and classic - it's not ostentatious. I listened to the accordionist and thought her interpretation was super. It is very different to the fuller, softer, airier tones associated with the harmonium. I thought I would prefer the accordion version by a mile - but I don't. The harmonium has texture, it is historic and feels authentic - and that counts for something.
The accordion is more precise and pure of tone, to my ear. The accordion, in many ways is overtaking a lot of older instruments. The musician has so much control due to the accordion bellows. And indeed I think Pigini is coming to define the classical accordion, more and more. To me it is balanced - neutral with warmth. I prefer its tone over the Russian bayan, where the low bass notes are very dominating. Along with the Gola and one or two other Italian free bass accordions (I do not wish to be exhaustive here) the Pigini sound is very sophisticated. There is a lovely balance between the hands and homogeneity of tone, that I associate more with quint converter than chromatic - perhaps due partly to the voice couplings on the bass - never see LL voice on quint. Though, perhaps there are other technical reasons - maybe due to the disposition of reeds on the bass. However, I think European C system bass with L.M. or L.M.H will create a lovely blend with the treble. Different instruments will have varying construction, so there is little point guessing. But it's good, as is the accordionist.
The only other thing I would mention, is that this classical accordion, has a tone that more reminds me of the Renaissance or early Baroque, whereas the harmonium, draws me more to the Romantic period. I don't know why, it's just does. If I were listening to Bach or Frescobaldi the accordion wins. Here though, to be frank, Franck on the harmonium might just edge it - by a whisker.
What do you think aemit?