Agreed with the others, in particular Palal. Casablanca is a corporate city. Yes there are some museums, ruins etc, but the main reason to go there is if you are on business. Marrakech is beautiful, lots to do, lots of culture, close to the desert - there is a reason it is so touristy - but it is very touristy. Having been to Morocco several times, I highly recommend Rabat, Tangier, Fes. It sounds like you are short on time, but Rabat is very close to Casablanca if you are flying into CMN.