We always rent through Costco because we find they have the better prices, flexibility, and you get a second driver for free.
I book the car when we make our air reservations. Then, I keep stalking the site for cheaper prices. When I find one, I rebook that car and then cancel the original reservation. Last year in Hilton Head I ended up saving over $300 and went from a mid-size car to a mid-size SUV. We were then upgraded to a huge SUV when we got there for free.
If you don't have a Costco membership, get one. It's worth it just for the car rental. When you get to Oahu it's a great place to load up on room supplies and eat the cheapest meal you'll have in Hawaii - the hot dog/soda deal.
ETA, also look at the flight price difference between Kona and Hilo. For us, it's usually significantly cheaper to fly into or out of one of the airports than the other. The cost difference in flights could make up for the increase in a one-way rental