Ah, yes, Sixti is only through the website. Believe it or not, but the website's actually good with availability. If it doesn't show up as available, then it's really not available. With Sixt (not Sixti), the cars are not pre-paid so people can be a no-show sometimes. There's more flexibility.

In fact, I have never seen a Sixti car in FRA. I guess they deliberately keep very few cars there! There's also mileage caps, and the car has to be clean when it's dropped off. They WILL nitpick over every single scratch, ding, and dents on the car.

If you want only Sixt, then the number I provided is good. Sixti has only three sizes, up to intermediate. Don't expect luxury or even premium cars. Also, their cars are all flashy with "Sixti" covered all over it. Free advertising for them, you rent it really cheap, it's a win-win.