Check carefully what is covered as part of the rental price. It may be "all in", covered 100%, no excess. Or it may be CDW only and you're liable for €1,000 or whatever sum they choose. If the former, then no need to take out extra cover. If the latter, then it's always cheaper to take out your own excess insurance beforehand (and not the one offered by the rental company) than the extortionate rates you'll be offered when you pick up the car. If you're likely to be doing this more often than a one-off, then an annual policy works out cheaper. As for which one is best, I've never had cause to make a claim, so can't answer that one.