With all due respect to Dan and not getting into the commissions discussions, I strongly recommend Expedia for the following reasons:
1) Possibility to cancel ONLINE until 11:59pm PST. (I've done it dozens [if not hundreds] of times)
2) Possibility to book an infant during the booking process without hassle.
3) Results offer options of 2 one-ways that may be cheaper
4) Earn rewards on top of airline miles (not worth that much, but PL offers nothing)
5) Possibility to earn "status" with them (spending $10k in 1 year or booking 15 nights at a hotel gets you gold - none are particularly hard for the typical family TA [pretty much every DDF'er]) with dedicated customer service lines. My personal experience with PL CS was terrible to say the least. It's extremely difficult to get a human being on the line, and when I finally figured out how to speak to someone, I was unimpressed with their service.
Orbitz is similar with a different rewards program (caveat is Orbucks expire after 12 months automatically). I don't have much personal experience with their CS though.
Bottom line, with no offense to anyone, I've yet to find any benefit that Priceline has over Expedia or Orbitz. And no, I'm NOT a paid advertiser for Expedia. I'm just relating my own personal experience in this without any bias of commissions etc.
p.s. And to give credit where it's due, thank you Dan for recently starting to give the option of Orbitz for most airfare deals which helps the majority of DDMS readers flying with babies, and not ONLY giving the PL option with the old argument that "everyone should be buying a seat for their infant anyway for safety and convenience reasons"
I forgot to mention another few benefits I've came across:
6) Expedia/Orbitz/Travelocity show you the fare class you're booking into which is helpful to calculate mileage earnings especially when crediting to partner programs.
7) Expedia recently started offering an insurance for a small fee, that if your ticket price drops after the time of purchase, they reimburse you the difference from what you paid.
8 ) Expedia includes results of select LCC that the PL doesn't like EasyJet.