I've been looking at car rental rates for Spain this summer. For the same class car (compact) and all the same travel info (dates, pick-up and drop-off locations), Kemwell has come in much lower than AutoEurope - about $300 less for a ten-day rental when insurance coverage and the additional driver fees are factored in. Not sure why, because both companies contract the actual rental with Europcar.

Last summer I booked through AutoEurope, but today I went ahead and booked with Kemwel - after my August trip, I will try to remember to come back to the forum and report whether there was any difference in service, etc.