Hertz - Using Hertz pts & upgrading

I"m planning on renting a Volvo or a Corvette in Maui next month and i'm making the booking on hertz points. As you know, you can normally upgrade to a prestige vehicle from about $25-$40/day (from a full-size) if they are available, I normally do since since I have president's circle status.

I currently only have 1500 points and these are the rates for booking for a week:

Fullsize: 2500
Regular Convertible: 4000
Corvette: 4500
Volvo: 7500

I"ll have to purchase points at $10 for 100 points. Based on this..which of the following should I do assuming I want the 2 better cars:

1) Book a full-size (Pay $100 for pts) --> Then pay the $25-$40/day extra to upgrade to the volvo or vette?

2) Book a regular convertible (Pay $250 for pts) ---> Would the upgrade to a volvo or vette be less then upgrading from a full size?

3) Book a Corvette (Pay $300 for pts) ---> If i decide to change at the airport to the Volvo..would it cost extra do you think?

I think most employees would gladly accept a $40 upgrade offer.