I always use a company like "holiday autos" they will search for the best deals from all the major companies like hertz, avis, etc and they are always cheaper than if you booked direct through these companies. Also if you book through a cashback website I use "topcashback" you can get some money back aswell.
Just beware of the extras when you collect, doesn't matter who you use there is always some sort of extra you have to pay for on collection
Enjoy your trip, I love going to Germany