Some more differences--
MSC has got many new ships--Meraviglia, Bellisima, Seaview, Seaside, Grandiosa, Virtuosa, Seashore, Seascape, Euribia, World Europa and even more being constructed;
Whereas Costa has only 4 new ships--Venezia, Toscana, Firenze & Smeralda.
MSC is right now the fastest growing cruise line and is expected to overtake both Royal Caribbean and Carnival in the near future.