Recently I've been hearing good things about sixT in Miami
It doesn't seem like
Sixt has any publicity at all on the forums. I know they don't have many locations in US, but they have an amazing offer for a
Mercedes GLK/ BMW X3/ Cadillac SRX for $49 a day (compared to general industry prices of $180 or so).
I booked 3 days with a car seat, express toll lanes capability (for the drive to Key West) for
$186 all in. Rest assured I checked with United Explorer CC that they cover insurance on this rental class as primary. THEY DO!