The only time to rent something other than a full size (which is a Camry, they go by EPA volume, not a reasonable idea of what full size would be to the average driver) is if you need a big SUV or larger vehicle for the seats/cargo.
Don't rent a luxury unless you know what they consider to be a luxury. Hertz will tell you exactly: Mercedes E-class, for example.
Negotiate an upgrade at the counter. I've rented Jaguars, Lexus, Mercedes, and other luxury cars for as little as $20/day over what I paid for the full size.
If you're part of their loyalty program, you'll often get a modest upgrade if it's available. E.g., rent a full size, get a new Impala, or Taurus (which are considered premium by Hertz). So, rent the full size, drive off in an SUV, or premium sedan.
I rent a car only 12 or 15 times a year now. Used to be more like 40 times a year.
Most recently, at Bradley airport (Hartford, CT) - where I rented a full size (Camry) and drove off in an Infiniti QX-60. Very nice car, by the way. I didn't rent it, or even negotiate it. They were just out of regular cars and had to choose who was getting a nicer car, so they upgraded a Gold club member.
To rent that car would cost about five times as much as the full size. So, rent the full size and be grateful when you get upgraded.
Or pay a LOT more and, in some cases, be dissatisfied if their idea of "luxury" doesn't meet yours.