Sorry, I am confused by your post in regards to the cost of the rental....
$7 x 9 days = $63 (w/Sunpass) or $10 x 9 days = $90 (w/o Sunpass)??
Unless I am missing some important information, (which I probably am), I would get the Sunpass option.
Having a Sunpass in Florida is a good idea. It can make car travel easier and quicker for many reasons (here are a few):
1. some toll roads do not have collection booths.
2. you do not have to worry about having cash for the booths.
3. you do not have to wait in traffic (if it is busy) to pay necessary tolls.
Will you be spending any time in Miami or surrounding areas? Or is this trip........ airport to the keys-keys back to the airport.