Why are you trying to pick between those specifically? IMHO unless you have specialist rental interests (Sixt is good for German cars) then Avis, Hertz and Alamo/National are all better and they are usually around the same price. With a standard rental I would favour Alamo/National as you get to pick your car from the reserved priced.
Be warned with SIXT, they have some bonkers geographical restrictions meaning you can't take the car out of California, Washington, Arizona or Nevada.
Be warned with SIXT, they have some bonkers geographical restrictions meaning you can't take the car out of California, Washington, Arizona or Nevada.