They are not equivalent. Europocar is a company that rents cars, like Hertz or Budget. AutoEurope is

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They are not equivalent. Europocar is a company that rents cars, like Hertz or Budget. AutoEurope is broker, an American company that sells reservations from other car companies; AutoEurope is like Travelocity or Orbitz. AutoEurope does not actually have cars that they own and rent out.
When I used AutoEurope hotel accommodation england in 2009 (rented in Zagreb, returned in Dubrovnik), they used a rental car company called Sixt (a big company like Hertz that is big in Europe). I think they had to mail you a voucher at that time - maybe they still do. Personally, I'd avoid the middleman if you can get the same price from Europcar.
By the way: unless you are taking a ferry, hotel accommodation england if you drive from Dubrovnik to Zagreb along the coast, you will have to drive through a tiny section of Bosnia and Herzegovina. You are required to have a special permit to drive your rental car from Croatia through Bosnia and Herzegovina; it should be free, but you need to request it when making you car reservation.
The coastal part of the drive is a lot like the Coast Highway in California, but then that part of Croatia looks a lot like California. The tiny section of Bosnia is really tatty. There are lots of apparently duty free stores where people come to buy cheap appliances and household goods as well as the usual duty free stuff.