Thanks!

How many separate rentals? Who's paying: employer? client? you?
> I won't be paying
I'll likely rent a car every week (for 4-5 days), or maybe keep the same car for 2 weeks

What are your priorities? Car upgrades? Maximizing FF miles? Earning free rental days for your personal use? Something else?
> Free rental days for personal use

Are you limited to just those three rental companies?
> Yes