In my experience you can usually do better booking directly with codes from Mousesavers (or sometimes even without the codes) if you are booking for a week or longer. If you are renting for just a few days sometimes Priceline is the best bet.
I would suggest checking out Mousesavers and playing with codes, etc and see what kind of rate you can get...and then make a lower bid on priceline and see if you get it. The big diff with Priceline is it is prepaid and cannot be changed....so you are locked in, and with a regular ressie you can cancel/change/rebook at will.
I have not had much luck lately with Priceline car rentals, but maybe you will have better luck.