Concerning prices (for compact and smarler cars) you are probably right.

There are some disadvantages using Sixti.
(1) There are only a few stations compared to "normal" Sixt service. If you arrive at FRA T1 it's quite troublesome to rent at Sixti as you have to go to T2 which is somewhat time consuming...

(2) If you want to have something else than a compact car, Sixti is not the way to go, as they only have cars up to compact. If you are lucky you can get a Mercedes A-class but that's it.

BTW If you book via internet more than 2 weeks in advance you have a good chance to get the same or even a better rates than plat