For a one or two day car rental, Hotwire is sometimes cheaper because Priceline adds a $5.95 processing charge to every winning bid. So that $16 bid on Priceline actually becomes about $22 (plus tax). But if you see a $20 offer on Hotwire, that's all you pay (plus tax).

Plus, you can easily knock $10 more with a voucher from milepoint.com (one-time only, cash in 500 miles from a major FF program).