For rental cars, I'm in the camp that feels like there really isn't a huge difference between the majors. My usual pattern is to just use Expedia to get comparison on rates, pick one I can live with and do reservation-only. (not prepaid) Then every week or so or when an airline or hotel chain sends you a discount code, do another price check.
Keep doing this up until departure time and make a new reservation if you find what seems like a good price. The rental car company's rental management system is constantly changing pricing based on what they think demand will be. Last year, my sister got caught up in a Delta IRROPS situation and ended up spending $17 to rent a sports car to get her from her diversion point in Nashville to Florida because Avis apparently needed the car down here and she essentially only paid the airport location taxes & fees for the reservation.