The lounge is definitely different since it was added two decades after the ship was built. When we first sailed Concierge on Magic, there was no lounge so the hosts had to find you around the ship if they wanted to offer you services. The lounge is smaller than the one on the Dream-class, but we prefer it because it has windows, and a direct access to a deck that overlooks the adult pool. There is plenty of shade on the deck, but no whirlpool. Inside the lounge, you'll find the same amenities : snacks, coffee machine and happy hour drinks.
Contrary to the Dream-class ships, the Concierge staterooms are located all over deck 8, so you might have to take a bit of a stroll to reach the lounge. You don't mention what stateroom category you are thinking of. All the rooms have roughly the same size as on the Dream-class ships - the 1BR is 8 square feet smaller on Magic. The verandah room have only a 2 sq ft difference. However, the 1 BR feels larger on Fantasy/Dream because of the layout.
If you are planning on sleeping 4 or 5, know that some 1BR suites on Magic/Wonder have the Pullman bed in the master BR. We do not find that very convenient since the Pullman is much more comfortable than the sofa bed.
Outside of that, the experience is pretty much the same.