I think its best to have a cancelable reservation with a rental company; periodically check the Hotwire rates and compare to to your rate; I would not make a purchase on Hotwire or Priceline until you know your plans are definite and you feel close enough to your travel date that an airline will not change your flight time, since the Hotwire and Priceline rates are prepaid and non-refundable; there is not set time when the best rates may be available on Hotwire or Priceline, but between 1 week and 1 month are the most likely times you find the best rates; the Hotwire listing will sometimes indicate if the counter and car are at the airport - if it is not stated, you will run the risk of a shuttle; I do not think there is any way tell on Priceline before you make the purchase whether the counter and car will be at the airport; ultimately, you will have to decide how much of a price difference is worth having to prepay, having the payment be non-refundable, and losing perks such as express lines with certain companies, reward points, etc.