It's a matter of taste, regarding both mechanics and sound.
The "domino coupler" model is the M series. The mechanics (and keyboard) may not be the greatest but some people really prefer the sound of this model over any other. The "domino couplers" sit very close to the keyboard, ensuring that they do not obstruct the sound coming out of the cassotto. The Morino IV or V M (the IV is generally preferred as it is less deep and also lighter) was first produced with Hohner Artiste reeds and later with Bugari reeds. The ones with the Bugari reeds are generally preferred. (I'm not sure whether this Bugari is related to the Bugari making accordions, but they are not the same in any case.)
The more common model is the Morino IV (or V) N which was made by Excelsior and most of these have Bugari reeds as well. (At some point Bugari stopped making reeds.) You could say that this is the "Golden era" for Morinos. The sound of the IV and V N is ever so slightly different from the later S series, a difference that can only be heard when you have them side by side. The S series has updated mechanics, and most importantly a grille fixed with decorative bolts (whereas the N series has levers that sometimes work themselves loose, causing the grille to fall off, preferably mid-concert). The S series has Cagnoni (tipo a mano) reeds.
So while the IV N may be the "golden" one you have to be careful to avoid getting a "Klebemorino". A few months of production of accordions around Castelfidardo used pallets that start sticking to the soundboard after a few months of use. And as the Hohner Morino (made by Excelsior) was very popular the majority of accordions with this defect seem to be Morinos (although there are accordions from other brands with the same defect). Replacing all the pallets was the only cure for the problem, but was not done on many of the affected accordions.
Around the year 2000 Pigini took over Excelsior, and the Morinos of the Pigini era (with black couplers) do not sound like the Excelsior-made Morinos (and even less like the earlier Hohner-made ones from the M series).
