I don't think there are any pros and many cons to using either one. Every single time I have tried to get a cheaper flight using one of those groups, I have been shut out. They let you go so far and then, just as you're ready to book and pay, you are told that seats are no longer available, but you can book 'these flights' for more money. For me, it's been all about bait and switch.
I book only through the official airline...they almost always have the cheapest fares. And if there's an issue, they can work with you to get it solved. Once you book through either Travelocity or Expedia, they don't much care. Hard to get any 'situations' resolved.