In general, Tokyo DisneySea is more for grownups, Tokyo Disneyland is more for families.
While this isn't the perfect comparison, Tokyo DisneySea is like Disney California Adventure while Tokyo Disneyland is more like Disneyland. DisneySea (like California Adventure) serves alcohol, has more of the "grownup" attractions like Indiana Jones and Tower of Terror, and is more about the beautiful settings than the traditional characters. I saw mostly adult couples at DisneySea, while Tokyo Disneyland was filled with families.