I've used both, but typically can get a better price via Priceline. The rentals are from the airport unless you specify a different location, so you would be getting a car from a well known rental agency. I have gotten cars from Hertz the most times. Recently I've gotten a Toyota Camry and a Chevy Impala (with 900 miles on it so brand new). One out of every 2 times I've gotten a free upgrade because they were out of the size I reserved.
I reserved a car for our Disney trip at the end of May for $12 per day on priceline. I think the lowest price for the standard (a midsized car with more bells & whistles) was $15.95 on Hotwire. I check that first and then start bidding a little lower on Priceline. If your bid gets rejected, you can change your class (size) of car and rebid, or wait 3 days and then rebid a dollar higher on the same size. Keep doing this until your bid gets accepted. I also notice that the farther out the date that I bid/reserve, the lower the price.
Oh, I forgot to add that the Entertainment book usually has a hotwire coupon, so check your book if you have one. Sometimes that helps hotwire be less than priceline. I got a $50 rebate back once from Hotwire using the coupon in the book.