My experiences are the opposite of what I read here. I usually rent from Europcar. A few years ago I also tried Sixt (because I got a Gold card from them through Hilton, and I felt that they generally had nicer cars), but I never got any further than checking their prices. In the end I always ended up renting from Europcar because of the price difference.

In Germany I only once had a bad experience with Europcar, at DUS, where I received a car I didn't like at all (something jeep-like from some American brand with a very "plastic toy" feeling). But I returned it after a 10 minutes and they offered me a Mercedes E-class for a very small surcharge instead. So in the end it turned out well...