Once the price quote is unveiled, you have 30 minutes to accept or reject it. On both Priceline and Hotwire, you get a discount only if you're content to buy a pig in a poke. The prices the two sites deliver are quite similar, at least on the sample trips I looked into. He wouldn't be likely to use Hotwire either. Whether Hotwire or Priceline is right for any given customer depends on that customer's needs.
Price is only one element of value.
Priceline does have one clear advantage over Hotwire, at least for the time being. The airlines backing Hotwire have bothered to build only an airline-reservation system. Unless you like sleeping on the street and taking taxis, you're probably going to gravitate to Priceline, where you can also name your own price on hotel rooms and rental cars. Perhaps that isn't the most obvious consideration for someone building a travel site to keep in mind. But I'd have trouble thinking of something more obvious.