When considering what form of currency to bring to Cuba, it is helpful to look back at a bit of history first, particularly the years 1991-1999. At that time in Cuba, there was an economic crisis known as “Período Especial” (special period). This rough time for the Caribbean Island was the result of two factors: the economic embargo put in place by the US government and the collapse of the communist countries in eastern Europe. In response, Fidel Castro decided toopen the gates to tourism to bring money from outside and create a second currency, creating a situation where Cubans would continue to use the Cuban Peso (at least for some very basic products) and the tourists would use American Dollars, allowing the economy to slowly grow.
Eventually, the “Peso Convertible Cubano” (CUP) was created, pushing the US dollar out of the Cuban finance system. Since the CUC was born to replace a foreign currency with the same value, the CUC and USD were exactly the same, just different notes. Eventually, in an attempt to fight back against the embargo, the dollar was devalued by 10%. In reality, the CUC and the USD move together always with a 10% difference.
At the time of this post 100 CUC = 90 USD and 100 USD = 87 CUC (money change tax). That is why I recommend coming to Cuba with euros. If you live in the US, change your money before coming.