When you go to the counter, politely ask for a Mustang Convertible (If that is the car of your choice) If there is one available, they will generally give it to you. Note that because there is one on the lot doesn't mean that it is available... There could be someone coming in later who paid extra to reserve the particular model, or that car could already be assigned through GC Carfirmation/Gold board. If there is nothing that you like in that class, you can request to pay for an upgrade into a higher class (whether that is a Prestige or Adrenaline collection, or another class) Again, if they have some cars available, they will offer it to you at a $x/day upgrade fee.. You can generally negotiate this, but it is all up to the agent...

I believe that the most common luxury cars Hertz carries now are Buick LaCrosse and Chrysler 300 (A 300 is usually a PCAR at other agencies last time I checked). Most PCARs are base model Maximas and Hyundai Genesis in stock. These are the most common for their respected class, but there are obviously other options that aren't as common in their fleet... Personally, I don't think that the premium to rent a PCAR or Luxury are worth it, and usually do an ICAR reservation, and use GC to get an F6 (usually available at the location I go to), although it might be good to reserve a SCAR with PC #159530 (which is a USAA PC I believe) which is a double upgrade coupon based on availability and trying to get a PCAR cheaper that way, although it is not guaranteed.

Ultimately, your experience will depend on the inventory levels when you go into the location, the agent you encounter, and luck ^