Something you should always consider: lighter means that the accordion (outer shell) is made out of thinner wood, which may not influence the sound in any significant way but which WILL influence the strength and thus the amount of damage in case of any accidents like bumps, fall, or worse, air travel in the cargo hold... It's not a matter of newer versus older accordion. A friend of mine recently had his Hohner Morino IV M (over 60 years old) tumble to the floor from a seat (so less than 2 feet, and not a straight drop). The treble case was broken in two corners and cracked all the way across the front (the side where it has the Hohner logo). The Morino M series is liked a lot because these accordions are lighter than the later Morino N and S series (made by Excelsior), and they also have a nicer sound. But few people really take into account how fragile the Morino M accordions are due to the thin outer shell. When you see a "lightweight" accordion, always consider that it will be more fragile than a heavier one of the same size!
People often think for instance that the Pigini Peter Pan is a great accordion for children (because it is small and very lightweight). But the thin plastic outer shell is fragile. The strap lugs may break off. The bass belt may break free from the housing, etc. This accordion may be great for a careful adult wanting to travel very light while carrying an accordion but this accordion is absolutely not suitable for a child (meaning careless, throwing their instrument around, etc.). The Peter Pan is a great example of why lightweight accordions may not always be a good idea.